AII amacrine cells in the distal inner nuclear layer of the mouse retina

Developmental errors in the common marmoset retina

Although retinal organization is remarkably conserved, morphological anomalies can be found to different extents and varieties across animal species with each presenting unique characteristics and patterns of displaced and misplaced neurons. One of the most widely used non-human primates in research, the common marmoset (Callithrix jaccus) could potentially also be of interest for visual research, but is unfortunately not well characterized in this regard. Therefore, the aim of our study was to provide a first time description of structural retinal layering including morphological differences and distinctive features in this species. Retinas from animals (n = 26) of both sexes and different ages were immunostained with cell specific antibodies to label a variety of bipolar, amacrine and ganglion cells. Misplaced ganglion cells with somata in the outermost part of the inner nuclear layer and rod bipolar cells with axon terminals projecting into the outer plexiform layer instead of the inner plexiform layer independent of age or sex of the animals were the most obvious findings, whereas misplaced amacrine cells and misplaced cone bipolar axon terminals occurred to a lesser extent. With this first time description of developmental retinal errors over a wide age range, we provide a basic characterization of the retinal system of the common marmosets, which can be taken into account for future studies in this and other animal species. The finding of misplaced ganglion cells and misplaced bipolar cell axon terminals was not reported before and displays an anatomic variation worthwhile for future analyzes of their physiological and functional impact.

Purinergic-Glycinergic Interaction in Neurodegenerative and Neuroinflammatory Disorders of the Retina

Neurodegenerative–neuroinflammatory disorders of the retina seriously hamper human vision. In searching for key factors that contribute to the development of these pathologies, we considered potential interactions among purinergic neuromodulation, glycinergic neurotransmission, and microglia activity in the retina. Energy deprivation at cellular levels is mainly due to impaired blood circulation leading to increased release of ATP and adenosine as well as glutamate and glycine. Interactions between these modulators and neurotransmitters are manifold. First, P2Y purinoceptor agonists facilitate reuptake of glycine by glycine transporter 1, while its inhibitors reduce reverse-mode operation; these events may lower extracellular glycine levels. The consequential changes in extracellular glycine concentration can lead to parallel changes in the activity of NR1/NR2B type NMDA receptors of which glycine is a mandatory agonist, and thereby may reduce neurodegenerative events in the retina. Second, P2Y purinoceptor agonists and glycine transporter 1 inhibitors may indirectly inhibit microglia activity by decreasing neuronal or glial glycine release in energy-compromised retina. These inhibitions may have a role in microglia activation, which is present during development and progression of neurodegenerative disorders such as glaucomatous and diabetic retinopathies and age-related macular degeneration or loss of retinal neurons caused by thromboembolic events. We have hypothesized that glycine transporter 1 inhibitors and P2Y purinoceptor agonists may have therapeutic importance in neurodegenerative–neuroinflammatory disorders of the retina by decreasing NR1/NR2B NMDA receptor activity and production and release of a series of proinflammatory cytokines from microglial cells.

Prox1 Is a Marker for AII Amacrine Cells in the Mouse Retina

The transcription factor Prox1 is expressed in multiple cells in the retina during eye development. This study has focused on neuronal Prox1 expression in the inner nuclear layer (INL) of the adult mouse retina. Prox1 immunostaining was evaluated in vertical retinal sections and whole mount preparations using a specific antibody directed to the C-terminus of Prox1. Strong immunostaining was observed in numerous amacrine cell bodies and in all horizontal cell bodies in the proximal and distal INL, respectively. Some bipolar cells were also weakly immunostained. Prox1-immunoreactive amacrine cells expressed glycine, and they formed 35 ± 3% of all glycinergic amacrine cells. Intracellular Neurobiotin injections into AII amacrine cells showed that all gap junction-coupled AII amacrine cells express Prox1, and no other Prox1-immunostained amacrine cells were in the immediate area surrounding the injected AII amacrine cell. Prox1-immunoreactive amacrine cell bodies were distributed across the retina, with their highest density (3887 ± 160 cells/mm²) in the central retina, 0.5 mm from the optic nerve head, and their lowest density (3133 ± 350 cells/mm²) in the mid-peripheral retina, 2 mm from the optic nerve head. Prox1-immunoreactive amacrine cell bodies comprised ~9.8% of the total amacrine cell population, and they formed a non-random mosaic with a regularity index (RI) of 3.4, similar to AII amacrine cells in the retinas of other mammals. Together, these findings indicate that AII amacrine cells are the predominant and likely only amacrine cell type strongly expressing Prox1 in the adult mouse retina, and establish Prox1 as a marker of AII amacrine cells.

Development of Retinal Amacrine Cells and Their Dendritic Stratification

Themammalian retina containsmultiple neurons, each of which contributes differentially to visual processing. Of these retinal neurons, amacrine cells have recently come to prime light since they facilitate majority of visual processing that takes place in the retina. Amacrine cells are also the most diverse group of neurons in the retina, classified majorly based on the neurotransmitter type they express and morphology of their dendritic arbors. Currently, little is known about the molecular basis contributing to this diversity during development. Amacrine cells also contribute to most of the synapses in the inner plexiform layer and mediate visual information input from bipolar cells onto retinal ganglion cells. In this review, we will describe the current understanding of amacrine cell and cell subtype development. Furthermore, we will address the molecular basis of retinal lamination at the inner plexiform layer. Overall, our review will provide a developmental perspective of amacrine cell subtype classification and their dendritic stratification.

Retinal distribution of Disabled-1 in a diurnal murine rodent, the Nile grass rat Arvicanthis niloticus

We sought to study the expression pattern of Disabled-1 (Dab1; an adaptor protein in the reelin pathway) in the cone-rich retina of a diurnal murine rodent. Expression was examined by western blotting and immunohistochemistry using well-established antibodies against Dab1 and various markers of retinal neurons. Western blots revealed the presence of Dab1 (80 kDa) in brain and retina of the Nile grass rat. Retinal immunoreactivity was predominant in soma and dendrites of horizontal cells as well as in amacrine cell bodies aligned at the INL/IPL border. Dab1(+) neurons in the inner retina do not stain for parvalbumin, calbindin, protein kinase C-alpha, choline acetyltransferase, glutamic acid decarboxylase, or tyrosine hydroxylase. They express, however, the glycine transporter GlyT1. They have small ovoid cell bodies (7.1 ± 1.06 μm in diameter) and bistratified terminal plexii in laminas a and b of the IPL. Dab1(+) amacrine cells are evenly distributed across the retina (2600 cells/mm(2)) in a fairly regular mosaic (regularity indexes ≈3.3-5.5). We conclude that retinal Dab1 in the adult Nile grass rat exhibits a dual cell patterning similar to that found in human. It is expressed in horizontal cells as well as in a subpopulation of glycinergic amacrine cells undetectable with antibodies against calcium-binding proteins. These amacrine cells are likely of the AII type.

Control of neuronal morphology by the atypical cadherin Fat3

Neurons receive signals through dendrites that vary widely in number and organization, ranging from one primary dendrite to multiple complex dendritic trees. For example, retinal amacrine cells (ACs) project primary dendrites into a discrete synaptic layer called the inner plexiform layer (IPL) and only rarely extend processes into other retinal layers. Here, we show that the atypical cadherin Fat3 ensures that ACs develop this unipolar morphology. AC precursors are initially multipolar but lose neurites as they migrate through the neuroblastic layer. In fat3 mutants, pruning is unreliable and ACs elaborate two dendritic trees: one in the IPL and a second projecting away from the IPL that stratifies to form an additional synaptic layer. Since complex nervous systems are characterized by the addition of layers, these results demonstrate that mutations in a single gene can cause fundamental changes in circuit organization that may drive nervous system evolution.

Guinea pig horizontal cells express GABA, the GABA-synthesizing enzyme GAD 65, and the GABA vesicular transporter

Gamma-aminobutyric acid (GABA) is likely expressed in horizontal cells of all species, although conflicting physiological findings have led to considerable controversy regarding its role as a transmitter in the outer retina. This study has evaluated key components of the GABA system in the outer retina of guinea pig, an emerging retinal model system. The presence of GABA, its rate-limiting synthetic enzyme glutamic acid decarboxylase (GAD(65) and GAD(67) isoforms), the plasma membrane GABA transporters (GAT-1 and GAT-3), and the vesicular GABA transporter (VGAT) was evaluated by using immunohistochemistry with well-characterized antibodies. The presence of GAD(65) mRNA was also evaluated by using laser capture microdissection and reverse transcriptase-polymerase chain reaction. Specific GABA, GAD(65), and VGAT immunostaining was localized to horizontal cell bodies, as well as to their processes and tips in the outer plexiform layer. Furthermore, immunostaining of retinal whole mounts and acutely dissociated retinas showed GAD(65) and VGAT immunoreactivity in both A-type and B-type horizontal cells. However, these cells did not contain GAD(67), GAT-1, or GAT-3 immunoreactivity. GAD(65) mRNA was detected in horizontal cells, and sequencing of the amplified GAD(65) fragment showed approximately 85% identity with other mammalian GAD(65) mRNAs. These studies demonstrate the presence of GABA, GAD(65), and VGAT in horizontal cells of the guinea pig retina, and support the idea that GABA is synthesized from GAD(65), taken up into synaptic vesicles by VGAT, and likely released by a vesicular mechanism from horizontal cells.

Immunocytochemical evidence for SNARE protein-dependent transmitter release from guinea pig horizontal cells

Horizontal cells are lateral interneurons that participate in visual processing in the outer retina but the cellular mechanisms underlying transmitter release from these cells are not fully understood. In non-mammalian horizontal cells, GABA release has been shown to occur by a non-vesicular mechanism. However, recent evidence in mammalian horizontal cells favors a vesicular mechanism as they lack plasmalemmal GABA transporters and some soluble NSF attachment protein receptor (SNARE) core proteins have been identified in rodent horizontal cells. Moreover, immunoreactivity for GABA and the molecular machinery to synthesize GABA have been found in guinea pig horizontal cells, suggesting that if components of the SNARE complex are expressed they could contribute to the vesicular release of GABA. In this study we investigated whether these vesicular and synaptic proteins are expressed by guinea pig horizontal cells using immunohistochemistry with well-characterized antibodies to evaluate their cellular distribution. Components of synaptic vesicles including vesicular GABA transporter, synapsin I and synaptic vesicle protein 2A were localized to horizontal cell processes and endings, along with the SNARE core complex proteins, syntaxin-1a, syntaxin-4 and synaptosomal-associated protein 25 (SNAP-25). Complexin I/II, a cytosolic protein that stabilizes the activated SNARE fusion core, strongly immunostained horizontal cell soma and processes. In addition, the vesicular Ca(2+)-sensor, synaptotagmin-2, which is essential for Ca(2+)-mediated vesicular release, was also localized to horizontal cell processes and somata. These morphological findings from guinea pig horizontal cells suggest that mammalian horizontal cells have the capacity to utilize a regulated Ca(2+)-dependent vesicular pathway to release neurotransmitter, and that this mechanism may be shared among many mammalian species.

Functional and structural modifications during retinal degeneration in the rd10 mouse

Mouse models of retinal degeneration are useful tools to study therapeutic approaches for patients affected by hereditary retinal dystrophies. We have studied degeneration in the rd10 mice both by immunocytochemistry and TUNEL-labeling of retinal cells, and through electrophysiological recordings. The cell degeneration in the retina of rd10 mice produced appreciable morphological changes in rod and cone cells by P20. Retinal cell death is clearly observed in the central retina and it peaked at P25 when there were 800 TUNEL-positive cells per mm(2). In the central retina, only one row of photoreceptors remained in the outer nuclear layer by P40 and there was a remarkable deterioration of bipolar cell dendrites postsynaptic to photoreceptors. The axon terminals of bipolar cells also underwent atrophy and the inner retina was subject to further changes, including a reduction and disorganization of AII amacrine cell population. Glutamate sensitivity was tested in rod bipolar cells with the single cell patch-clamp technique in slice preparations, although at P60 no significant differences were observed with age-matched controls. Thus, we conclude that rod and cone degeneration in the rd10 mouse model is followed by deterioration of their postsynaptic cells and the cells in the inner retina. However, the functional preservation of receptors for photoreceptor transmission in bipolar cells may open new therapeutic possibilities.

Identification of molecular markers of bipolar cells in the murine retina

Retinal bipolar neurons serve as relay interneurons that connect rod and cone photoreceptor cells to amacrine and ganglion cells. They exhibit diverse morphologies essential for correct routing of photoreceptor cell signals to specific postsynaptic amacrine and ganglion cells. The development and physiology of these interneurons have not been completely defined molecularly. Despite previous identification of genes expressed in several bipolar cell subtypes, molecules that mark each bipolar cell type still await discovery. In this report, novel genetic markers of murine bipolar cells were found. Candidates were initially generated by using microarray analysis of single bipolar cells and mining of retinal serial analysis of gene expression (SAGE) data. These candidates were subsequently tested for expression in bipolar cells by RNA in situ hybridization. Ten new molecular markers were identified, five of which are highly enriched in their expression in bipolar cells within the adult retina. Double-labeling experiments using probes for previously characterized subsets of bipolar cells were performed to identify the subtypes of bipolar cells that express the novel markers. Additionally, the expression of bipolar cell genes was analyzed in Bhlhb4 knockout retinas, in which rod bipolar cells degenerate postnatally, to delineate further the identity of bipolar cells in which novel markers are found. From the analysis of Bhlhb4 mutant retinas, cone bipolar cell gene expression appears to be relatively unaffected by the degeneration of rod bipolar cells. Identification of molecular markers for the various subtypes of bipolar cells will lead to greater insights into the development and function of these diverse interneurons.

Displaced amacrine cells of the mouse retina

The aim of this study was to characterize and classify the displaced amacrine cells in the mouse retina. Amacrine cells in the ganglion cell layer were injected with fluorescent dyes in flat-mounted retinas. Dye-filled displaced amacrine cells were classified according to dendritic field size, horizontal and vertical stratification patterns, and general morphology. We identified 10 different morphological types of displaced amacrine cell. Six of the cell types identified here are novel cell types that have not been described previously in the mouse retina, to the best of our knowledge. The displaced amacrine cells included four types of medium-field cells, with dendritic field diameters of 200-500 microm, and six types of wide-field cells, with dendritic fields extending over 500 microm. Narrow-field displaced amacrine cells, with dendritic field diameters smaller than 200 microm, were not encountered. The most frequently labeled displaced amacrine cell type was the starburst amacrine cell. At least three cell types identified here have nondisplaced counterparts in the inner nuclear layer as well. Displaced amacrine cells display a rich variety of stratification and branching patterns, which surely reflect the wide range of their functional roles in the processing of visual signals in the inner retina.