General-purpose and special-purpose visual systems

The information that eyes supply supports a wide variety of functions, from the guidance systems that enable an animal to navigate successfully around the environment, to the detection and identification of predators, prey, and conspecifics. The eyes with which we are most familiar the single-chambered eyes of vertebrates and cephalopod molluscs, and the compound eyes of insects and higher crustaceans allow these animals to perform the full range of visual tasks. These eyes have evidently evolved in conjunction with brains that are capable of subjecting the raw visual information to many different kinds of analysis, depending on the nature of the task that the animal is engaged in. However, not all eyes evolved to provide such comprehensive information. For example, in bivalve molluscs we find eyes of very varied design (pinholes, concave mirrors, and apposition compound eyes) whose only function is to detect approaching predators and thereby allow the animal to protect itself by closing its shell. Thus, there are special-purpose eyes as well as eyes with multiple functions

Silicon retina with adaptive photoreceptors

The central problem faced by the retina is to encode reliably small local differences in image intensity over a several-decade range of background illumination. The distal layers of the retina adjust the transducing elements to make this encoding possible. Several generations of silicon retinae that integrate phototransducers and CMOS processing elements in the focal plane are modeled after the distal layers of the vertebrate retina. A silicon retina with an adaptive photoreceptor that responds with high gain to small spatial and temporal variations in light intensity is described. Comparison with a spatial and temporal average of receptor response extends the dynamic range of the receptor. Continuous, slow adaptation centers the operating point of the photoreceptor around its time-average intensity and compensates for static transistor mismatch

Lack of the Sodium-Driven Chloride Bicarbonate Exchanger NCBE Impairs Visual Function in the Mouse Retina

Regulation of ion and pH homeostasis is essential for normal neuronal function. The sodium-driven chloride bicarbonate exchanger NCBE (Slc4a10), a member of the SLC4 family of bicarbonate transporters, uses the transmembrane gradient of sodium to drive cellular net uptake of bicarbonate and to extrude chloride, thereby modulating both intracellular pH (pHi) and chloride concentration ([Cl-]i) in neurons. Here we show that NCBE is strongly expressed in the retina. As GABAA receptors conduct both chloride and bicarbonate, we hypothesized that NCBE may be relevant for GABAergic transmission in the retina. Importantly, we found a differential expression of NCBE in bipolar cells: whereas NCBE was expressed on ON and OFF bipolar cell axon terminals, it only localized to dendrites of OFF bipolar cells. On these compartments, NCBE colocalized with the main neuronal chloride extruder KCC2, which renders GABA hyperpolarizing. NCBE was also expressed in starburst amacrine cells, but was absent from neurons known to depolarize in response to GABA, like horizontal cells. Mice lacking NCBE showed decreased visual acuity and contrast sensitivity in behavioral experiments and smaller b-wave amplitudes and longer latencies in electroretinograms. Ganglion cells from NCBE-deficient mice also showed altered temporal response properties. In summary, our data suggest that NCBE may serve to maintain intracellular chloride and bicarbonate concentration in retinal neurons. Consequently, lack of NCBE in the retina may result in changes in pHi regulation and chloride-dependent inhibition, leading to altered signal transmission and impaired visual function

Investigating the roles of Hedgehog signalling in the developing lamina of the Drosophila melanogaster visual system

Neurodevelopment is controlled by complex and tightly controlled mechanisms. They function to ensure neuronal diversity at the right time and location. These processes can be activated non-autonomously when surrounding cells release biochemical signals, for example, Hedgehog (Hh) in Drosophila melanogaster. Hh signalling is frequently required during neurodevelopment where it regulates cell specification and numbers. The Drosophila melanogaster visual system is comprised of the retina and the optic lobe, which is further divided into four processing layers: lamina, medulla, lobula and lobula plate. During the development of the visual system, Hh signalling induces photoreceptor differentiation in the eye disc (developing retina). Consequently, photoreceptors transmit Hh to the optic lobe to the outer proliferation centre neuroepithelium. Photoreceptor-derived Hh induces lamina precursor cell (LPC) specification, terminal divisions and characteristic lamina column formation. My thesis re-examined the role of Hh signalling in early lamina development (prior to and up to column formation) and late lamina development (after column assembly). Overall, I discovered multiple instances where LPCs developed independently of Hh signalling, where the only intervention was blocking cell death. Secondly, I investigated which signal could be required to specify LPC fate. I found that blocking the expression of Aop, a transcriptional repressor as part of the MAPK cascade, blocked lamina development. Finally, I investigated how lamina neuron specification is controlled during late lamina development. Inducing high and low levels of Hh signalling activity gives rise to different lamina neuron fates. Furthermore, I found that Hh protein was distributed in a gradient. This work is exciting since the precise mechanisms that go into gradient formation are yet to be understood fully. It has been reported that Hh signalling is disrupted in several rat models of neurodegeneration. My findings implicate the genetically tractable Drosophila melanogaster lamina as a model to study Hh in survival and as a morphogen

The cell biology of vision

Humans possess the remarkable ability to perceive color, shape, and motion, and to differentiate between light intensities varied by over nine orders of magnitude. Phototransduction—the process in which absorbed photons are converted into electrical responses—is the first stage of visual processing, and occurs in the outer segment, the light-sensing organelle of the photoreceptor cell. Studies of genes linked to human inherited blindness have been crucial to understanding the biogenesis of the outer segment and membrane-trafficking of photoreceptors

General features of the retinal connectome determine the computation of motion anticipation

Motion anticipation allows the visual system to compensate for the slow speed of phototransduction so that a moving object can be accurately located. This correction is already present in the signal that ganglion cells send from the retina but the biophysical mechanisms underlying this computation are not known. Here we demonstrate that motion anticipation is computed autonomously within the dendritic tree of each ganglion cell and relies on feedforward inhibition. The passive and non-linear interaction of excitatory and inhibitory synapses enables the somatic voltage to encode the actual position of a moving object instead of its delayed representation. General rather than specific features of the retinal connectome govern this computation: an excess of inhibitory inputs over excitatory, with both being randomly distributed, allows tracking of all directions of motion, while the average distance between inputs determines the object velocities that can be compensated for

PhD

dissertationThe mammalian retina is comprised of 55-60 cell types mediating transduction of photic information through visual preprocessing channels. These cell types fall into six major cell superclasses including photoreceptors, horizontal, amacrine, Muller and ganglion cells. Through computational molecular phenotyping, using amino acids as discriminands, this dissertation shows that the major cellular superclasses of the murine retina are subdivisible into the following natural classes; 1 retinal pigment epithelium class, 2 photoreceptor, 2 bipolar cell, 1 horizontal cell, 15 amacrine cell, 1 Muller cell, and 7 ganglion cell classes. Retinal degenerative diseases like retinitis pigmentosa result in loss of photoreceptors, which constitutes deafferentation of the neural retina. This deafferentation, when complete, is followed by retinal remodeling, which is the common fate of all retinal degenerations that trigger photoreceptor loss. The same strategy used to visualize cell classes in wild type murine retina was applied to examples of retinal degenerative disease in human tissues and naturally and genetically engineered models, examining all cell types in 17 human cases of retinitis pigmentosa (RP) and 85 cases of rodent retinal degenerations encompassing 13 different genetic models. Computational molecular phenotyping concurrently visualized glial transformations, neuronal translocations, and the emergence of novel synaptic complexes, achievements not possible with any other method. The fusion of phenotyping and anatomy at the ultrastructure level also enabled the modeling of synaptic connections, illustrating that the degenerating retina produces new synapses with vigor with the possibility that this phenomenon might be exploited to rescue vision. However, this circuitry is likely corruptive of visual processing and reflects, we believe, attempts by neurons to find synaptic excitation, demonstrating that even minor rewiring seriously corrupts signal processing in retinal pathways leaving many current approaches to bionic and biological retinal rescue unsustainable. The ultimate conclusion is that the sequelae of retinal degenerative disease are far more complex than previously believed, and schemes to rescue vision via bionic implants or stem/engineered cells are based on presumed beliefs in preservation of normal wiring and cell population patterning after photoreceptor death. Those beliefs are incorrect: retinal neurons die, migrate, and create new circuitries. Vision rescue strategies will need to be refined

PhD

dissertationThe mammalian retina is comprised of 55-60 cell types mediating transduction of photic information through visual preprocessing channels. These cell types fall into six major cell superclasses including photoreceptors, horizontal, amacrine, Muller and ganglion cells. Through computational molecular phenotyping, using amino acids as discriminands, this dissertation shows that the major cellular superclasses of the murine retina are subdivisible into the following natural classes; 1 retinal pigment epithelium class, 2 photoreceptor, 2 bipolar cell, 1 horizontal cell, 15 amacrine cell, 1 Muller cell, and 7 ganglion cell classes. Retinal degenerative diseases like retinitis pigmentosa result in loss of photoreceptors, which constitutes deafferentation of the neural retina. This deafferentation, when complete, is followed by retinal remodeling, which is the common fate of all retinal degenerations that trigger photoreceptor loss. The same strategy used to visualize cell classes in wild type murine retina was applied to examples of retinal degenerative disease in human tissues and naturally and genetically engineered models, examining all cell types in 17 human cases of retinitis pigmentosa (RP) and 85 cases of rodent retinal degenerations encompassing 13 different genetic models. Computational molecular phenotyping concurrently visualized glial transformations, neuronal translocations, and the emergence of novel synaptic complexes, achievements not possible with any other method. The fusion of phenotyping and anatomy at the ultrastructure level also enabled the modeling of synaptic connections, illustrating that the degenerating retina produces new synapses with vigor with the possibility that this phenomenon might be exploited to rescue vision. However, this circuitry is likely corruptive of visual processing and reflects, we believe, attempts by neurons to find synaptic excitation, demonstrating that even minor rewiring seriously corrupts signal processing in retinal pathways leaving many current approaches to bionic and biological retinal rescue unsustainable. The ultimate conclusion is that the sequelae of retinal degenerative disease are far more complex than previously believed, and schemes to rescue vision via bionic implants or stem/engineered cells are based on presumed beliefs in preservation of normal wiring and cell population patterning after photoreceptor death. Those beliefs are incorrect: retinal neurons die, migrate, and create new circuitries. Vision rescue strategies will need to be refined

The Homodimeric Kinesin, Kif17, is Essential for Vertebrate Photoreceptor Sensory Outer Segment Development

Sensory cilia and intraflagellar transport (IFT), a pathway essential for ciliogenesis, play important roles in embryonic development and cell differentiation. In vertebrate photoreceptors IFT is required for the early development of ciliated sensory outer segments (OS), an elaborate organelle that sequesters the many proteins comprising the phototransduction machinery. As in other cilia and flagella, heterotrimeric members of the kinesin 2 family have been implicated as the anterograde IFT motor in OS. However, in Caenorhabditis elegans, OSM-3, a homodimeric kinesin 2 motor, plays an essential role in some, but not all sensory cilia. Kif17, a vertebrate OSM-3 homologue, is known for its role in dendritic trafficking in neurons, but a function in ciliogenesis has not been determined. We show that in zebrafish Kif17 is widely expressed in the nervous system and retina. In photoreceptors Kif17 co-localizes with IFT proteins within the OS, and co-immunoprecipitates with IFT proteins. Knockdown of Kif17 has little if any effect in early embryogenesis, including the formation of motile sensory cilia in the pronephros. However, OS formation and targeting of the visual pigment protein is severely disrupted. Our analysis shows that Kif17 is essential for photoreceptor OS development, and suggests that Kif17 plays a cell type specific role in vertebrate ciliogenesis

Neuronal circuitry of the pigeon retina (Columba livia) : the morphological classification and organization of various neuronal types

The three studies presented in this thesis were conducted to advance our understanding of the retinal circuitry that contributes to processing high visual acuity in the pigeon. In the first study, the topographic density changes and degree of photoreceptor (PR) to retinal ganglion cell (RGC) convergence in the pigeon retina was determined. DAPI or Propidium iodide labelled PRs and RGCs were counted in the retina. Rod density was quantified by counting anti-rod opsin stained outer segments. The fovea and the red field contained significantly higher cone and RGC densities compared with the yellow field. Rods were missing from the fovea, but not in the red field, which suggests that a rod circuitry may be present in this area. The ratio of cones to RGCs was lower in both the fovea and red field, which is consistent with the higher visual acuities that have been reported in these regions. The second study classified the types of DiO-Iabelled bipolar cells in the fovea, central red and yellow fields. Eight bipolar cell types were classified in the retina using a modification of Mariani's (1987) classification scheme. Eight BC types (B1 - B8) had similar dendritic morphology as the ones described by Mariani. Two bipolar cell types, B7 and B8, had comparatively smaller dendritic fields than the other types. It was estimated to receive input from possibly one photoreceptor in the fovea and the central red field. Based on the small dendritic field size, B7 and B8 may be good candidates for being the midget-like BCs in the pigeon retina. The third study classified the RGC groups in the pigeon retina. Classification of RGCs labelled with DiI/DiO in the pigeon retina was based on the dendritic stratification pattern in the inner plexiform layer (IPL). Five morphological RGC groups were identified, the unstratified, monostratified, bistratified, tristratified and tetrastratified. The unstratified group was characterised by vertically oriented dendrites occupying a thick portion of the IPL, whereas the other groups had horizontally oriented dendrites stratifying at narrow portion of the IPL. The unstratified RGC had the narrowest dendritic field (diameter >- 18.5 urn). Based on the unstratified RGC's dendritic field size, it is a good candidate for a 'midget-like' RGC in the pigeon retina. However, it has a different morphology from the primate midget RGC. Further work is required to determine the physiology and differential distribution of the different types of bipolar and ganglion cells in the pigeon retina