Müller glial cells contribute to dim light vision in the spectacled caiman (Caiman crocodilus fuscus): Analysis of retinal light transmission.

In this study, we show the capability of Müller glial cells to transport light through the inverted retina of reptiles, specifically the retina of the spectacled caimans. Thus, confirming that Müller cells of lower vertebrates also improve retinal light transmission. Confocal imaging of freshly isolated retinal wholemounts, that preserved the refractive index landscape of the tissue, indicated that the retina of the spectacled caiman is adapted for vision under dim light conditions. For light transmission experiments, we used a setup with two axially aligned objectives imaging the retina from both sides to project the light onto the inner (vitreal) surface and to detect the transmitted light behind the retina at the receptor layer. Simultaneously, a confocal microscope obtained images of the Müller cells embedded within the vital tissue. Projections of light onto several representative Müller cell trunks within the inner plexiform layer, i.e. (i) trunks with a straight orientation, (ii) trunks which are formed by the inner processes and (iii) trunks which get split into inner processes, were associated with increases in the intensity of the transmitted light. Projections of light onto the periphery of the Müller cell endfeet resulted in a lower intensity of transmitted light. In this way, retinal glial (Müller) cells support dim light vision by improving the signal-to-noise ratio which increases the sensitivity to light. The field of illuminated photoreceptors mainly include rods reflecting the rod dominance of the of tissue. A subpopulation of Müller cells with downstreaming cone cells led to a high-intensity illumination of the cones, while the surrounding rods were illuminated by light of lower intensity. Therefore, Müller cells that lie in front of cones may adapt the intensity of the transmitted light to the different sensitivities of cones and rods, presumably allowing a simultaneous vision with both receptor types under dim light conditions.

Unidirectional photoreceptor-to-Müller glia coupling and unique K+ channel expression in Caiman retina.

BACKGROUND: Müller cells, the principal glial cells of the vertebrate retina, are fundamental for the maintenance and function of neuronal cells. In most vertebrates, including humans, Müller cells abundantly express Kir4.1 inwardly rectifying potassium channels responsible for hyperpolarized membrane potential and for various vital functions such as potassium buffering and glutamate clearance; inter-species differences in Kir4.1 expression were, however, observed. Localization and function of potassium channels in Müller cells from the retina of crocodiles remain, hitherto, unknown. METHODS: We studied retinae of the Spectacled caiman (Caiman crocodilus fuscus), endowed with both diurnal and nocturnal vision, by (i) immunohistochemistry, (ii) whole-cell voltage-clamp, and (iii) fluorescent dye tracing to investigate K+ channel distribution and glia-to-neuron communications. RESULTS: Immunohistochemistry revealed that caiman Müller cells, similarly to other vertebrates, express vimentin, GFAP, S100ß, and glutamine synthetase. In contrast, Kir4.1 channel protein was not found in Müller cells but was localized in photoreceptor cells. Instead, 2P-domain TASK-1 channels were expressed in Müller cells. Electrophysiological properties of enzymatically dissociated Müller cells without photoreceptors and isolated Müller cells with adhering photoreceptors were significantly different. This suggests ion coupling between Müller cells and photoreceptors in the caiman retina. Sulforhodamine-B injected into cones permeated to adhering Müller cells thus revealing a uni-directional dye coupling. CONCLUSION: Our data indicate that caiman Müller glial cells are unique among vertebrates studied so far by predominantly expressing TASK-1 rather than Kir4.1 K+ channels and by bi-directional ion and uni-directional dye coupling to photoreceptor cells. This coupling may play an important role in specific glia-neuron signaling pathways and in a new type of K+ buffering.

Müller cell alignment in bird fovea: possible role in vision.

Birds which possess high visual acuity, such as eagles and falcons, are known to have retinas with a deep conically curved central foveal pit. There have been different attempts to explain the importance of this particular shape of the fovea in visual resolution. Recently, the function of Müller cells as "light fibers" was discovered, showing how the endfeet of Müller cells trap the light and then transfer it to a single cone photoreceptor. Here we describe how the endfeet of Müller cells line the walls of the foveal pit in the Pied Flycatcher, and how the Müller cell body extends its processes towards individual cones, forming machinery that could allow for light transfer from the pit wall to the photoreceptor layer alongside the pit. We describe how this construction may send an image from the fovea to the cones, and also, how the angular positioning of Müller cells, being optical extensions of the cones, has the advantage of being much denser than on a flat or slightly curved fovea. We, therefore, suggest that this type of optic fiber alignment can be used as a novel type of "amplifying array" that simply increases the amount of megapixels at the photoreceptor cell layer.

Effects of intravitreal bevacizumab (Avastin) on the porcine retina.

BACKGROUND: To evaluate the effects of intravitreal bevacizumab (Avastin) on the porcine retina, with respect to structural alterations, expression of proteins involved in apoptosis (bax, caspase-3, caspase-9) and gliosis (vimentin, GFAP), expression of factors which influence the development of vascular edema (VEGF, PEDF), and of membrane channels implicated in retinal osmohomeostasis (Kir4.1, aquaporin-1, aquaporin-4). METHODS: One eye of seven adult pigs received a single intravitreal injection of bevacizumab (1.25 mg). Control eyes received buffered saline. For light and electron microscopy, the eyes were prepared 3 (one animal) and 7 days (two animals) after injection. Retinal slices were immunostained against gliosis- and apoptosis-related proteins. The gene expression was determined in the neuroretina and the retinal pigment epithelium of the remaining four animals with real-time RT-PCR 2 days after injection of bevacizumab. RESULTS: Intravitreal bevacizumab did not induce alterations in the retinal structure, neither at light microscopic nor at electron microscopic level. The photoreceptors were well-preserved; no signs of photoreceptor damage or mitochondrial swelling were observed. Bevacizumab did also not induce reactive gliosis (as indicated by the unaltered immunolocalization of the glial proteins vimentin, GFAP, and glutamine synthetase) or apoptosis (as indicated by the unaltered immunolocalization of bax, caspase-3, and caspase-9). Intravitreal bevacizumab decreased the transcriptional expression of VEGF-A, and increased the expression of Kir4.1 in the neuroretina and pigment epithelium, and of PEDF in the pigment epithelium. Bevacizumab did not alter the transcriptional expression of GFAP, bax, caspase-3, VEGF receptor-1 and -2, and aquaporin-1 and -4. CONCLUSIONS: A single intravitreal injection of bevacizumab does not result in structural changes of the porcine retina, nor in induction of gliosis or apoptosis. The bevacizumab-induced transcriptional downregulation of VEGF and upregulation of Kir4.1 might protect the retina from the development of vascular and cytotoxic edema.