Vertebrate Visual System Research Articles

Six3-mediated auto repression and eye development requires its interaction with members of the Groucho-related family of co-repressors.

Recent findings suggest that Six3, a member of the evolutionarily conserved So/Six homeodomain family, plays an important role in vertebrate visual system development. However, little is known about the molecular mechanisms by which this function is accomplished. Although several members of the So/Six gene family interact with members of the eyes absent (Eya) gene family and function as transcriptional activators, Six3 does not interact with any known member of the Eya family. Here, we report that Grg4 and Grg5, mouse counterparts of the Drosophila transcriptional co-repressor Groucho, interact with mouse Six3 and its closely related member Six6, which may also be involved in vertebrate eye development. The specificity of the interaction was validated by co-immunoprecipitation of Six3 and Grg4 complexes from cell lines. We also show that the interaction between Six3 and Grg5 requires the Q domain of Grg5 and a conserved phenylalanine residue present in an eh1-like motif located in the Six domain of Six3. The pattern of Grg5 expression in the mouse ventral forebrain and developing optic vesicles overlapped that previously reported for Six3 and Six6. Using PCR, we identified a specific DNA motif that is bound by Six3 and we demonstrated that Six3 acts as a potent transcriptional repressor upon its interaction with Groucho-related members. We also demonstrated that this interaction is required for Six3 auto repression. The biological significance of this interaction in the retina and lens was assessed by overexpression experiments using either wild type full-length Six3 cDNA or a mutated form of this gene in which the interaction with Groucho proteins was disrupted. Overexpression of wild type Six3 by in vivo retroviral infection of newborn rat retinae led to an altered photoreceptor phenotype, while the in ovo electroporation of chicken embryos resulted in failure of lens placode invagination and production of delta-crystallin-negative cells within the placode. These specific alterations were not seen when the mutated form of Six3 cDNA was used in similar experimental approaches, indicating that Six3 interaction with Groucho proteins plays an essential role in vertebrate eye development.

Dopamine and nitric oxide control both flickering and steady-light-induced cone contraction and horizontal cell spinule formation in the teleost (carp) retina: serial interaction of dopamine and nitric oxide.

Adaptation to ambient light, which is an important characteristic of the vertebrate visual system, involves cellular and subcellular (synaptic) plasticity of the retina. The present study investigated dopamine (DA) and nitric oxide (NO) as possible neurochemical modulators controlling cone photomechanical movements (PMMs) and horizontal cell (HC) spinules in relation to steady and flickering light adaptation in the carp retina. Haloperidol (HAL; a nonspecific DA receptor blocker) or cPTIO (a NO scavenger) largely inhibited the cone PMMs and HC spinule formation induced by either steady or flickering light. These results suggested that both DA and NO could be involved in the light-adaptation changes induced by either pattern of input and that DA and NO effects may not be completely independent. The possibility that NO and DA interact serially was evaluated pharmacologically by cross-antagonist application (i.e., DA + cPTIO or NO + HAL). When a NO donor was coapplied with HAL to dark-adapted eyecups, normal light-adaptive cone PMMs and HC spinules occurred. In contrast, when DA was applied in the presence of cPTIO, the dark-adapted state persisted. It was concluded 1) that DA and NO are both light-adaptive neurochemicals, released in the retina during either steady or flickering light; 2) that the effects of DA and NO on light-adaptive cone PMMs and HC spinules do not occur in parallel; and 3) that NO and DA act mainly in series, specifically as follows: Light --> DA --> NO --> Cone PMMs + HC spinules.

Eph Receptor Tyrosine Kinase-Mediated Formation of a Topographic Map in theDrosophilaVisual System

Roles for Eph receptor tyrosine kinase signaling in the formation of topographic patterns of axonal connectivity have been well established in vertebrate visual systems. Here we describe a role for a Drosophila Eph receptor tyrosine kinase (EPH) in the control of photoreceptor axon and cortical axon topography in the developing visual system. Although uniform across the developing eye, EPH is expressed in a concentration gradient appropriate for conveying positional information during cortical axon guidance in the second-order optic ganglion, the medulla. Disruption of this graded pattern of EPH activity by double-stranded RNA interference or by ectopic expression of wild-type or dominant-negative transgenes perturbed the establishment of medulla cortical axon topography. In addition, abnormal midline fasciculation of photoreceptor axons resulted from the eye-specific expression of the dominant-negative EPH transgene. These observations reveal a conserved role for Eph kinases as determinants of topographic map formation in vertebrates and invertebrates.

Differential expression of cadherin-2 and cadherin-4 in the developing and adult zebrafish visual system

Cadherins are homophilic cell adhesion molecules that control development of a variety of tissues and maintenance of adult structures. Although cadherins have been implicated in the development of the brain, including the visual system, in several vertebrate species, little is known of their role in zebrafish. In this study, we examined distribution of cadherin-2 (Cdh2, N-cadherin) in the visual system of developing and adult zebrafish using both immunocytochemical and in situ hybridization methods, and we compared Cdh2 distribution to that of the previously reported and closely related cadherin-4 (Cdh4, R-cadherin). As in other vertebrates, in zebrafish embryos Cdh2 was widely expressed in the early nervous system, but its expression became more restricted as development proceeded. Cdh4 was not detectable until later in development, at about the time when the first ganglion cells are generated. Cdh2 and Cdh4 were expressed in distinct regions of developing visual structures, including the lens. We hypothesize that the differential expression of these two cadherins in developing zebrafish visual structures reflects functionally different roles in the development of the vertebrate visual system.

Axonal Regeneration in the Central Nervous System

Ingoglia N. A., Murray M. (Eds)Axonal Regeneration in the Central Nervous SystemMarcel Dekker, New York, 2000, 709 pages. Price $225. ISBN 0-82-470410-X Currently, there is much optimism regarding the prospects for imminent development of a rational and effective therapy for spinal cord injury, prompting the question as to whether researchers in this field have always felt that therapeutic breakthroughs are ‘just around the corner’. The Foreword to this book (written by Bernice Grafstein) addresses this question by summarizing the ‘half a century of regeneration research’ since the book ‘Central Nervous System Regeneration’ was published in 1955. The tone is then set for expositions on various approaches to understanding regeneration and summaries of key experiments, thus forming a coherent ‘snapshot’ of the current state of regeneration research. The interesting point is made that some of the findings now regarded as central to our understanding of the organization and response to injury of the CNS were not initially regarded with much enthusiasm. For effective regeneration several processes must occur appropriately, these include: removal of degeneration products, axonal regeneration, target finding, and, finally, restitution of previously existing circuitry. Each of these components is dealt with in different sections of the book. The book is divided into four major parts, dealing primarily with regeneration in different physiological circumstances and the various manipulations of the environment and neurones that are required for regeneration. The regenerative response in the lower vertebrate visual system and during mammalian development are discussed, although the bulk of the text deals with spontaneous responses to axotomy of adult mammalian neurones and strategies designed to alter axonal growth. At many stages there is repetition of the importance of the balance between the intrinsic regenerative capabilities of the neurone and the axonal environment, however this repetition is to be expected for such a fundamental concept. Each chapter is written from a different standpoint, this allows each author to stress particular aspects of their work that they feel is important, consequently this aids the reader in understanding new directions that each field may take. For instance the importance of rapid axonal sealing is stated in many chapters throughout the book in different contexts. Manipulation of the growth responses and the differences between axonal sprouting and regenerative growth have been highlighted in many recent studies, hence the difference between these modes of growth forms the focus of some of the later chapters. Illustrating the breadth of scope of this book are the chapters towards the end that discuss what will be required for clinical trials on regeneration therapies, and, in one instance (Reier et al.) even describe some preliminary results of clinical treatment. In all, although it is difficult for a volume of this type to remain completely up to date for very long, this book can be recommended highly as a means of keeping abreast of the progress made so far in this important and fascinating subject. Coverage of the information across so wide a sphere means that a very large amount of information on the multifaceted world of axonal regeneration has been distilled into an easily digestible form. A minor gripe would be the poor reproduction of some illustrations, in particular the absence of colour in the axonal sealing figures. In answer to the question implicitly posed in the Foreword there appears truly to have been substantial progress towards the development of rational therapy for CNS injury, although clearly many of the details will require much further work. However, a point made in several chapters is that since there are so many obstacles to effective axonal regeneration in the mammalian CNS there might be some evolutionary reason to suppose that it is inappropriate in free-living individuals – perhaps a reason that is not currently appreciated. It is interesting to speculate on how the book's content will be viewed in a further 50 years.

ON–OFF retinal ganglion cells temporally encode OFF/ON sequence

While the functions of ON and OFF retinal ganglion cells have been intensively investigated, that of ON–OFF cells has not. In the present study, the temporal properties of spike trains emitted from ON-OFF cells in response to randomly flickering or multiphase ramp stimuli were examined in the Japanese quail. The results indicate that the firing of ON-spikes was influenced by the recent firing of OFF-spikes, and vice versa. As a result of this interaction, OFF/ON sequence of light intensity change was encoded with a spike pair with an interval of 20 ms, indicating that temporal coding is utilized in the vertebrate visual system as early as the retina. Thus, the present results suggest that retinal neuronal circuits may detect specific sequential features of stimuli.

Zebrafish mutants: behavioral genetic studies of visual system defects.

Zebrafish are a promising model for behavioral and genetic studies of vertebrate visual system development and retinal degeneration. In the past few years, numerous studies on zebrafish vision have been published. While most of the studies focus on the molecular and cellular characterization of mutations that disrupt zebrafish visual system structure in early development, others examine the mechanisms that underlie inherited visual system disorders in adults. Behavioral assays, along with morphologic and electrophysiological methods, are powerful tools for functional analyses of zebrafish visual development and performance.

Making connections. Meeting: axon guidance and neural plasticity.

Meeting on ‘Axon guidance and neural plasticity’ held at the Cold Spring Harbor Laboratory, NY, September 20–24, 2000 (organized by Tobias Bonhoeffer, Marc Tessier‐Lavigne and Larry Zipursky). ![][1] It was just over a century ago that Santiago Ramon y Cajal first marvelled at the intricate patterns of neuronal connectivity in the nervous system. Finding out how these connections are established has intrigued and challenged generations of developmental neuroscientists. It is, however, only within the last decade that advances in molecular biology, genetics, imaging and electrophysiology have combined to allow the first insights into the molecular and cellular mechanisms that control the formation and refinement of neuronal connections. To provide a forum for the exchange of information and ideas in this rapidly expanding field, Cold Spring Harbor Laboratory hosts a meeting on ‘Axon guidance and neural plasticity’ every two years. The most recent meeting attracted some 400 participants, 265 of whom presented the results of their latest research. Here we review just a small selection of these presentations, highlighting some of the major new themes that emerged. New mechanisms of axon guidance continue to be discovered at a rapid pace (Table 1). This trend shows no signs of abating. Two highlights of the Cold Spring Harbor meeting were talks reporting the identification of new guidance mechanisms. One of these is implicated in retinal axon targeting in the vertebrate visual system; the other in the inhibition of axon regeneration in the mature mammalian CNS. View this table: Table 1. Some of the molecules that guide axons Ever since the pioneering work of Roger Sperry in the 1950s and ‘60s, the retinotectal system has been an informative model system for both theoretical and experimental studies of axon pathfinding. Retinal axons project topographically to the tectum (or its mammalian equivalent, the superior colliculus): nasal retinal axons map to posterior and temporal axons to anterior tectum (Figure 1A … [1]: /embed/graphic-1.gif

Rhodopsin: structural basis of molecular physiology.

The crystal structure of rod cell visual pigment rhodopsin was recently solved at 2.8-A resolution. A critical evaluation of a decade of structure-function studies is now possible. It is also possible to begin to explain the structural basis for several unique physiological properties of the vertebrate visual system, including extremely low dark noise levels as well as high gain and color detection. The ligand-binding pocket of rhodopsin is remarkably compact, and several apparent chromophore-protein interactions were not predicted from extensive mutagenesis or spectroscopic studies. The transmembrane helices are interrupted or kinked at multiple sites. An extensive network of interhelical interactions stabilizes the ground state of the receptor. The helix movement model of receptor activation, which might apply to all G protein-coupled receptors (GPCRs) of the rhodopsin family, is supported by several structural elements that suggest how light-induced conformational changes in the ligand-binding pocket are transmitted to the cytoplasmic surface. The cytoplasmic domain of the receptor is remarkable for a carboxy-terminal helical domain extending from the seventh transmembrane segment parallel to the bilayer surface. Thus the cytoplasmic surface appears to be approximately the right size to bind to the transducin heterotrimer in a one-to-one complex. Future high-resolution structural studies of rhodopsin and other GPCRs will form a basis to elucidate the detailed molecular mechanism of GPCR-mediated signal transduction.

Pax genes in development and maturation of the vertebrate visual system: implications for optic nerve regeneration.

Pax genes play a pivotal role in development of the vertebrate visual system. Pax6 is the master control gene for eye development: ectopic expression of Pax6 in Xenopus laevis and Drosphila melanogaster leads to the formation of differentiated eyes on the legs or wings. Pax6 is involved in formation of ganglion cells of the retina, as well as cells of the lens, iris and cornea. In addition Pax6 may play a role in axon guidance in the visual system. Pax2 regulates differentiation of the optic disk through which retinal ganglion cell axons exit the eye. Furthermore, Pax2 plays a critical role in development of the optic chiasm and in the guidance of axons along the contralateral or ipsilateral tracts of the optic nerve to visual targets in the brain. During development Pax7 is expressed in neuronal cells of one of the major visual targets in the brain, the optic tectum/superior colliculus. Neurons expressing Pax7 migrate towards the pia and concentrate in the stratum griseum superficiale (SGFS), the target site for retinal axons. Together, expression of Pax2, 6 and 7 may guide axons during formation of functional retinotectal/collicular projections. Highly regulated Pax gene expression is also observed in mature animals. Moreover, evidence suggests that Pax genes are important for regeneration of the visual system. We are currently investigating Pax gene expression in species that display a range of outcomes of optic nerve regeneration. We predict that such information will provide valuable insights for the induction of successful regeneration of the optic nerve and of other regions of the central nervous system in mammals including man.

Photoreceptor light adaptation. Untangling desensitization and sensitization.

The illumination at the earth's surface varies by >10 orders of magnitude during the normal day–night cycle, and the vertebrate visual system covers this entire range of light intensities with two neuronal subsystems that rely on the activity of two types of photoreceptor cells, rods and cones.

Theory of spatiochromatic image encoding and feature extraction.

We consider how to interpret, filter, and cross-correlate complex-value color (hue and saturation) images by using a single discrete Fourier transform: the spatiochromatic discrete Fourier transform. The model defines new types of spatiochromatic oriented sinusoidal gratings, termed rainbow gratings, which encode the variation of color over space. We demonstrate how color-opponent detectors observed within the vertebrate visual system can be easily defined by linear filters within this representation. This model also allows us to filter and detect both spatial and chromatic patterns in images by using a single cross-correlation procedure. In doing so, we explore a new form of the Cauchy-Schwartz inequality applied to complex-valued scalar products. Results demonstrate the power of this form of spatiochromatic matched filtering in detecting signals embedded in such a significant amount of noise that they are not visible to the unaided human eye.

Filopodia, spines, and the generation of synaptic diversity.

The above studies appear to be generally consistent with the idea that dendrite dynamics and, consequently, patterns of neural connectivity are modulated by electrical activity. Given these facts, it still remains to be determined what this dynamism means to the organism. How large of a role do filopodia play in synaptogenesis in the developing brain? Is the refinement of topographic maps dependent on active dendritic filopodia? How are synapses stabilized? What are the interactions between axonal and dendritic arbors? What are the cellular and molecular differences responsible for generating a spine versus a filopodium? What are the specific functional and behavioral consequences of these cellular and molecular differences? The fundamental issue here, as elsewhere, is: What goes on in an organism when nobody is watching? Which small scale processes are important for a given large scale process and which are merely happening at the same time in a causally unrelated way? As is beginning to occur (11xJontes, J.D, Buchanan, J, and Smith, S.J. Nat. Neurosci. 2000; 3: 231–237Crossref | PubMed | Scopus (143)See all References, 13xLendvai, B, Stern, E.A, Chen, B, and Svoboda, K. Nature. 2000; 404: 876–881Crossref | PubMed | Scopus (491)See all References), such issues can be addressed by using minimally invasive techniques to watch cellular and molecular dynamics during the normal development and functioning of intact organisms.*To whom correspondence should be addressed (e-mail: sjsmith@leland.stanford.edu).

A visible difference

A visible difference

Embryonic Lens Repels Retinal Ganglion Cell Axons

During development of the vertebrate visual system, retinal ganglion cell (RGC) axons follow a precise path toward their midbrain targets. Although much is known about the cues that direct RGC axons once they have left the optic disc, less is known about the guidance of axons at earlier stages, when RGCs first send out their axons to navigate within the developing retina. Using collagen gel coculture experiments, we find that the embryonic lens produces a powerful diffusible repulsive activity for RGC axons. We also find that this activity is localized to the lens epithelium and not the lens fiber layer, while the pigmented epithelium and vitreous humour are devoid of activity. The further observation that the lens also chemorepels primary sensory axons, but does not repel olfactory bulb axons, shows that this activity is specific for subsets of axons. Our experiments have excluded two candidate repellents for RGC axons (collapsin-1/sema III and chondroitin sulfate proteoglycans). These results implicate the lens in the earliest stages of RGC axon guidance. One function of the lens repellent may be to prevent aberrant targeting toward the lens, and it may also be involved in the directional guidance of RGC axons toward the optic disc.

Cloning and characterization of six zebrafish photoreceptor opsin cDNAs and immunolocalization of their corresponding proteins

Zebrafish (Danio rerio) represents an excellent genetic model for vertebrate visual system studies. Because the opsin proteins are ideal markers of specific photoreceptor cell types, we cloned six different zebrafish opsin cDNAs. Based on pairwise alignments and phylogenetic comparisons between the predicted zebrafish opsin amino acid sequences and other vertebrate opsins, the cDNAs encode rhodopsin, two different green opsins (zfgr1 and zfgr2), a red, a blue, and an ultraviolet opsin. Phylogenetic analysis indicates the zfgr1 protein occupies a well-resolved dendrogram branch separate from the other green opsins examined, while zebrafish ultraviolet opsin is closely related to the human blue- and chicken violet-sensitive proteins. Polyclonal antisera were generated against individual bacterial fusion proteins containing either the red, blue, or ultraviolet amino termini or the rod or green opsin carboxyl termini. Immunolocalization on adult zebrafish frozen sections demonstrates the green and red opsins are each expressed in different members of the double cone cell pair, the blue opsin is detected in long single cones, and the ultraviolet opsin protein is expressed in the short single cones. In 120-h postfertilization wholemounts, green, red, blue, and ultraviolet opsin-positive cells are detected in an orderly arrangement throughout the entire retina. The antibodies' photoreceptor-type specificity indicates they will be useful for characterizing both wild-type and mutant zebrafish retinas.

Neurotrophins in the developing and regenerating visual system.

The neurotrophins NGF, BDNF, NT-3 and NT-4 have a wide range of effects in the development and regeneration of neural circuits in the visual system of vertebrates. This review focuses on the localization and functions of neurotrophins in the retina, lateral geniculate nucleus, suprachiasmatic nucleus, superior colliculus/optic tectum, and isthmic nuclei. Research of the past 20 years has shown that neurotrophins and their receptors are localized in numerous visual centers from the retina to the visual cortex, and that neurotrophins influence proliferation, neurite outgrowth and survival of cells in the visual system in vitro and in vivo. A relationship between electrical activity and neurotrophic functions has been established in several visual centers in the CNS, and neurotrophins have been implicated in synaptic plasticity in the visual cortex. Besides functions of neurotrophins as retrograde, target-derived trophic factors, recent data indicate that neurotrophins may have anterograde, afferent as well as local, paracrine actions in the retina, optic nerve and the visual cortex. Some neurotrophins appear to regulate proliferation and survival of glial cells in the optic pathways. Neurotrophins increase the survival of retinal ganglion cells after axotomy or ischemia and they promote the regeneration of retinal ganglion cell axons in some vertebration. Neurotrophins also rescue photoreceptors from degeneration. These findings implicate the neurotrophins not only as important regulators during development, but also as potential therapeutic agents in degenerative retinal diseases and after optic nerve injury.

Mechanisms of Cell Death in the Inherited Retinal Degenerations

The human retina possesses both extraordinary sensitivity and a huge adaptive range. A light flash containing only a few photons is sufficient to be consciously perceived in humans. We also can see over a range of nearly nine orders of magnitude in background illumination. The vertebrate visual system has been pushed, by evolutionary pressures, to approach its thermodynamic limitations. This exquisite tuning, however, comes at a cost. Photoreceptors are forced to carry large metabolic burdens, to renew continuously their light-sensitive outer segments and to exist under conditions of very high pO2. Modest changes in the biochemical environment or cellular structure of photoreceptors, caused by genetic lesions or environmental insults, are sufficient to induce cell death through apoptosis. For the inherited retinal degenerations, understanding the impact of each molecular defect on the biology of photoreceptors is important, in addition to studying how these effects converge upon the apoptotic pathway. Collectively, these studies should lead to the development of rational therapies that may slow or even reverse the progression of this devastating disease process.

不可思議な気体性伝達物質ＮＯ：その現状と展望 視覚系における機能を中心にして

The discovery of nitric oxide (NO) as an intercellular messenger brought about new concepts regarding the induction of synaptic plasticity in the neural circuits. Its importance has been recognized since the first enthusiasm, though certain inconsistent results have been reported recently on the effects of NO. In this review, latest studies are surveyed focusing on the functions of NO in vertebrate visual systems. There, this simple gas plays a role of adaptation signal in the retina, and it also acts as an important messenger involved in the self-organization of the optic nerve projection during the development.

Chapter 15 Development of the Retina

As in other vertebrate species, the zebrafish retina is simpler than other regions of the central nervous system. This relative simplicity along with rapid development, and accessibility to genetic analysis make the zebrafish retina an excellent model system for studies of neurogenesis in the vertebrate CNS. Several genetic screens have led to the isolation of an impressive collection of mutants affecting the retina and the retinotectal projections in zebrafish. A variety of techniques and markers are available to study the isolated mutants. These include several antigen- and transcript-detection methods, retrograde and anterograde labeling of neurons, blastomere transplantations, H3 labeling, and others. As past genetic screens have achieved a rather low level of saturation, the current collection of mutants can only grow in the future. Morphological and behavioral criteria have been successfully applied in zebrafish to search for defects in spinal development. In future genetic screens, progressively more sophisticated screening approaches will make it possible to detect very subtle changes in the retinal development. The remarkable evolutionary conservation of the vertebrate eye provides the basis for using the zebrafish as a model system for the detection and analysis of genetic defects potentially related to human eye disorders. Some of the genetic defects of the zebrafish retina indeed resemble human retinopathies. As the genetic analysis of the vertebrate visual system is far from being complete and new techniques are being introduced at a rapid pace, the zebrafish embryo will become increasingly useful as a model for studies of the vertebrate retina.