Monoclonal antibodies distinguish subtypes of retinal horizontal cells

Retinal horizontal cells of goldfish (Carassius auratus) display subtype-specific differences in spontaneous action potentials in situ

Horizontal cells (HCs) are neurons of the outer retina, which provide inhibitory feedback onto photoreceptors and contribute to image processing. HCs in teleosts are classified into four subtypes (H1-H4), each having different roles: H1-H3 feed back onto different sets of cones, H4 feed back onto rods, and only H1 store and release the inhibitory neurotransmitter, γ-aminobutyric acid (GABA). Dissociated HCs exhibit spontaneous Ca²⁺ -based action potentials (APs), yet it is unclear if APs occur in situ, or if all subtypes exhibit APs. We measured intracellular Ca²⁺ and report APs in slice preparations of the goldfish retina. In HCs furthest from photoreceptors (i.e., H3/H4), APs were less frequent, with greater duration and area under the curve (a measure of Ca²⁺ flux). Next, we classified acutely dissociated HCs into subtypes by integrating the ratio of dendritic field size vs. soma size (r_d/s ). H1 and H2 subtypes had low r_d/s values (<8); H3/H4 had high r_d/s (>12). To verify this model, H1s were identified by immunoreactivity for GABA and 95% of these cells had an r_d/s < 4. In Ca²⁺ imaging experiments, as r_d/s increased, AP duration and area under the curve increased, while frequency decreased. Our results demonstrate the presence of Ca²⁺ -based APs in the goldfish retina in situ and show that HC subtypes H1 through H4 exhibit progressively longer and less frequent spontaneous APs. These results suggest that APs may play an important role in inhibitory feedback, and may have implications for understanding the relative contributions of HC subtypes in the outer retina.

The zebrafish young mutation acts non-cell-autonomously to uncouple differentiation from specification for all retinal cells

Embryos from mutagenized zebrafish were screened for disruptions in retinal lamination to identify factors involved in vertebrate retinal cell specification and differentiation. Two alleles of a recessive mutation, young, were isolated in which final differentiation and normal lamination of retinal cells were blocked. Early aspects of retinogenesis including the specification of cells along the inner optic cup as retinal tissue, polarity of the retinal neuroepithelium, and confinement of cell divisions to the apical pigmented epithelial boarder were normal in young mutants. BrdU incorporation experiments showed that the initiation and pattern of cell cycle withdrawal across the retina was comparable to wild-type siblings; however, this process took longer in the mutant. Analysis of early markers for cell type differentiation revealed that each of the major classes of retinal neurons, as well as non-neural Muller glial cells, are specified in young embryos. However, the retinal cells fail to elaborate morphological specializations, and analysis of late cell-type-specific markers suggests that the retinal cells were inhibited from fully differentiating. Other regions of the nervous system showed no obvious defects in young mutants. Mosaic analysis demonstrated that the young mutation acts non-cell-autonomously within the retina, as final morphological and molecular differentiation was rescued when genetically mutant cells were transplanted into wild-type hosts. Conversely, differentiation was prevented in wild-type cells when placed in young mutant retinas. Mosaic experiments also suggest that young functions at or near the cell surface and is not freely diffusible. We conclude that the young mutation disrupts the post-specification development of all retinal neurons and glia cells.

Organization and development of horizontal cells in the goldfish retina, I: The use of monoclonal antibody AT101

We have produced and characterized a monoclonal antibody, AT101, which selectively labels both viable and formaldehyde-fixed horizontal cell axon terminals, but not their somas or axons, of the goldfish (Carassius auratus) retina. The antigen recognized by AT101 appears to be a cell surface glycoprotein with a molecular weight of about 35,000 Daltons, and is present exclusively or predominantly in nervous tissues of all vertebrate species examined. We have used AT101 as a probe to analyze immunocytochemically the organization of horizontal cell axon terminals (HCATs) in the adult goldfish retina, and the emergence and maturation of these terminals during retinal development. Because of continued growth at the retinal margin in adult goldfish, there is a peripheral-to-central gradient in the age of cells, with the most mature in the center and the youngest in the periphery. In the center and near periphery of the adult retina, HCATs have a fusiform morphology and form a dense network in the middle and proximal part of the inner nuclear layer. In the far peripheral retina, the axon terminals appear round or ellipsoid. The retina closest to the retinal margin is devoid of AT101 staining, indicating that either HCATs are absent or the antigen recognized by AT101 is not present on HCATs at this stage. A similar sequence of changes in staining pattern is seen during development. Although AT101 staining can first be demonstrated in the larval retina at 1 month after hatching, it appears mostly as punctate structures. At a later stage, there are round or ellipsoid structures that resemble in morphology and location (in the inner nuclear layer) those found in the far peripheral adult retina. Double-labeling experiments with AT101 and antiserum against tubulin also indicate that AT101 labels the HCATs when they first appear during development. These data suggest that the emergence and maturation of HCAT is a late event in retinal development.

In vivo expression of perforin by natural killer cells during a viral infection. Studies on uveitis produced by herpes simplex virus type I

A potent cytolytic pore-forming protein (PFP, perforin, or cytolysin) is associated with the cytoplasmic granules of cytotoxic T lymphocytes (CTL) and natural killer (NK) cells. The role of PFP/perforin in cytolytic reactions carried out in vivo is still unclear. Here, the authors performed immunohistochemical analysis using antibodies monospecific for perforin and made use of a murine uveitis model produced by intracameral inoculation of herpes simplex virus I (HSV-I). The main cell infiltrate found in the anterior segment of virus-inoculated eyes consisted of Thy-1+/asialo GM1+/CD8-/CD4- cells, presumably representing NK cells. Perforin staining was detected mainly in cells bearing this phenotype. Perforin was only detected in cells displaying the large granular lymphocyte morphology. A small number of perforin-positive cells (less than 5%) colabeled as CD8+, indicating that these cells could have belonged to the CTL lineage. These observations show for the first time the presence of perforin-containing NK cells in tissues of animals undergoing acute viral infections.

A monoclonal antibody marker for the paraboloid region of cone photoreceptors in turtle retina

Monoclonal antibodies that specifically label one or more cell types in retina have been produced; however, only a few antibodies that, in addition, recognize distinct subcellular structures in these cells have been reported. During a search for monoclonal antibodies that bind to specific cell types in the turtle (Pseudemys scripta elegans) retina, we obtained an antibody (20 93; an IgG) that labels the inner segment of cone photoreceptors. Ultrastructural immunocytochemistry using immunogold and avidin/biotin-peroxidase techniques showed that 20 93 antigen is localized to the paraboloid, a region specifically involved in glycogen metabolism in cones. In addition, a few bipolar cells were found to be labeled. The monoclonal antibody showed limited species cross-reactivity and failed to stain mouse, rat, rabbit, dog, cow, Anolis, and human retinas. Immunoblotting showed that monoclonal antibody 20-93 binds to a 40 KDa protein that is present in the retinal membrane. The antibody should be useful in immunological studies of the cone paraboloid.

In vivo expression of perforin by CD8+ lymphocytes during an acute viral infection

CTL and NK cells cultured in vitro have been shown to contain a cytolytic pore-forming protein (PFP/perforin/cytolysin). To date, it has not been determined whether perforin is expressed by CTL that have been primed in vivo. Here, we have infected mice with two strains of lymphocytic choriomeningitis virus (LCMV), one of which mainly produces choriomeningitis and, the other, hepatitis. Brain and liver cryostat sections obtained from LCMV-infected mice were stained for various lymphocyte markers, including perforin. We were able to detect a large accumulation of perforin antigen in CD8+/Thy-1+/asialo GM1+/CD4- lymphocytes, which in fact represent the main infiltrating cell type found in brain and liver sections obtained during the late acute stage of LCMV infection. Perforin was also detected in a smaller population of CD8-/asialo GM1+/NK 1.1+/F480- cells, presumably corresponding to NK cells. Perforin-positive cells were found to have the morphology of blasts or large granular lymphocytes (LGL). These observations, together with in vitro studies performed in the past, indicate that perforin may be associated exclusively with LGL-like CTL blasts and NK cells. Our results demonstrate for the first time the presence of perforin in CTL that have been primed in vivo and suggest that perforin-positive CTL may be directly involved in producing the immunopathology associated with the LCMV infection.

Synapse formation in retina is influenced by molecules that identify cell position

Molecules that identify cell type and position in the nervous system were detected by monoclonal antibodies. One molecule, TOP, is distributed in a 35-fold topographic gradient from the dorsoposterior margin to the ventroanterior margin of avian retina. The gradient is present in young embryos, increases with retinal growth, and persists in the adult. TOP molecules are present on most or all cells of retina. The number of TOP molecules detected per cell varies continuously along the axis of the antigen gradient. Thus, TOP can be used to identify position in the plane of retina along that axis. Other antigens that identify cell type and position across the thickness of retina also were detected. Molecules that mark such cellular organization may represent a neuronal recognition system. Antibodies were used to examine the role of markers of cell position in development of the nervous system. Antibody to TOP from hybridoma cells that were injected into in vivo embryo eyes diffused into the retina and bound in a topographic gradient of Ab.TOP complexes. Synapse formation in retina was inhibited in the presence of anti-TOP antibody. This suggests that TOP is involved in synapse formation and that recognition of position by neurons is necessary for normal synapse formation.