Retinal bipolar cell input mechanisms in giant danio. I. Electroretinographic analysis

Dynamic functional and structural remodeling during retinal regeneration in zebrafish

Introduction

Zebrafish regenerate their retinas following damage, resulting in restoration of visual function. Here we evaluate recovery of retinal function through qualitative and quantitative analysis of the electroretinogram (ERG) over time following retinal damage, in correlation to histological features of regenerated retinal tissue.

Methods

Retinas of adult zebrafish were lesioned by intravitreal injection of 10 μM (extensive lesion; destroys all neurons) or 2 μM (selective lesion; spares photoreceptors) ouabain. Unlesioned contralateral retinas served as controls. Function of retinal circuitry was analyzed at selected timepoints using ERG recordings from live zebrafish, and whole eyes were processed for histological analyses immediately thereafter.

Results

Qualitative and quantitative assessment of waveforms during retinal regeneration revealed dynamic changes that were heterogeneous on an individual level within each sampling time, but still followed common waveform recovery patterns on a per-fish and population-level basis. Early in the regeneration period (13-30 days post injury; DPI), for both lesion types, b-waves were essentially not detected, and unmasked increased apparent amplitudes, implicit times, and half-widths of a-waves (vs. controls). In control recordings, d-waves were not obviously detected, but apparent d-waves (OFF-bipolar responses) from regenerating retinas of several fish became prominent by 30DPI and dominated the post-photoreceptor response (PPR). Beyond 45DPI, b-waves became detectable, and the ratio of apparent d- to b-wave contributions progressively shifted with most, but not all, fish displaying a b-wave dominated PPR. At the latest timepoints (extensive, 90DPI; selective, 80DPI), recordings with measurable b-waves approached a normal waveform (implicit times and half-widths), but amplitudes were not restored to control levels. Histological analyses of the retinas from which ERGs were recorded showed that as regeneration progressed, PKCa + ON-bipolar terminals and parvalbumin + amacrine cell processes became more stereotypically positioned within the deep sublaminae of the INL over recovery time after each lesion type, consistent with the shift in PPR seen in the ERG recordings.

Discussion

Taken together, these data suggest that photoreceptor-OFF-bipolar component/connectivity may functionally recover and mature earlier during regeneration compared to the photoreceptor-ON-bipolar component, though the timeframe in which such recovery happens is heterogeneous on a per-fish basis. Collectively our studies suggest gradual restoration of ON-bipolar functional circuitry during retinal regeneration.

Grk7 but not Grk1 undergoes cAMP-dependent phosphorylation in zebrafish cone photoreceptors and mediates cone photoresponse recovery to elevated cAMP

In the vertebrate retina, phosphorylation of photoactivated visual pigments in rods and cones by G protein-coupled receptor kinases (GRKs) is essential for sustained visual function. Previous in vitro analysis demonstrated that GRK1 and GRK7 are phosphorylated by PKA, resulting in a reduced capacity to phosphorylate rhodopsin. In vivo observations revealed that GRK phosphorylation occurs in the dark and is cAMP dependent. In many vertebrates, including humans and zebrafish, GRK1 is expressed in both rods and cones while GRK7 is expressed only in cones. However, mice express only GRK1 in both rods and cones and lack GRK7. We recently generated a mutation in Grk1 that deletes the phosphorylation site, Ser21. This mutant demonstrated delayed dark adaptation in mouse rods but not in cones in vivo, suggesting GRK1 may serve a different role depending upon the photoreceptor cell type in which it is expressed. Here, zebrafish were selected to evaluate the role of cAMP-dependent GRK phosphorylation in cone photoreceptor recovery. Electroretinogram analyses of larvae treated with forskolin show that elevated intracellular cAMP significantly decreases recovery of the cone photoresponse, which is mediated by Grk7a rather than Grk1b. Using a cone-specific dominant negative PKA transgene, we show for the first time that PKA is required for Grk7a phosphorylation in vivo. Lastly, immunoblot analyses of rod grk1a-/- and cone grk1b-/- zebrafish and Nrl-/- mouse show that cone-expressed Grk1 does not undergo cAMP-dependent phosphorylation in vivo. These results provide a better understanding of the function of Grk phosphorylation relative to cone adaptation and recovery.

Antagonistic surround responses in different cones are mediated by feedback synapses from different horizontal cells

Cone photoreceptors are the first neurons along the visual pathway that exhibit center-surround antagonistic receptive fields, the basic building blocks for spatial information processing in the visual system. The surround responses in cones are mediated by the horizontal cells (HCs) via multiple feedback synaptic mechanisms. It has been controversial on which mechanisms are responsible for the surround-elicited depolarizing responses in cones (ΔVCone(s)), and whether the surround responses of various types of cones are mediated by the same HC feedback mechanisms. In this report, we studied ΔVCone(s)) of four types of cones in the salamander retina, and found that they are mediated by feedback synapses from A-type, B-type or A- and B-type HCs. ΔVCone(s) are observable in the presence of concomitant center light spots, and surround + center light stimuli of various intensity, size and wavelength differentially activate the feedback synapses from A- and B-type HCs to cones. We found that ΔVCone(s) of the L-cones are mediated by both A- and B-type HCs, those of the P- and S-cones by B-type HCs, and those of the A-cones by the A-type HCs. Moreover, our results suggest that B-type HCs mediate ΔVCone(s) through both GABAergic and GluT-ClC feedback synaptic mechanisms, and A-type HCs mediate ΔVCone(s) via the GluT-ClC feedback mechanism. Feedback synaptic mechanisms that increase calcium influx in cone synaptic terminals play important roles in mediating the antagonistic surround responses in the postsynaptic bipolar cells, but they may not generate enough current to depolarize the cones and significantly contribute to ΔVCone(s).

Electroretinogram analysis of zebrafish retinal function across development

PURPOSE

The electroretinogram (ERG) is a powerful approach for investigating visual function in zebrafish ocular disease models. However, complexity, cost, and a literature gap present as significant barriers for the introduction of this technology to new zebrafish laboratories. Here, we introduce a simplified and effective method to obtain zebrafish ERGs.

METHODS

In-house assembled recording electrodes and a custom 3D-printed platform were used to gather high-quality and consistent ERG data from zebrafish at 3 developmental timepoints-larval, juvenile, and adult. Fish were tested under both scotopic (dark-adapted) and photopic (light-adapted) conditions to differentiate between the rod and cone systems, respectively.

RESULTS

Robust ERG waveforms across all developmental timepoints were obtained using the methodology presented here. We observed an overall increase in signal amplitude as development progressed, reflecting maturation of the zebrafish retina. Oscillatory potentials could also be isolated from the generated waveforms.

CONCLUSIONS

This simplified approach to the zebrafish ERG can generate waveforms comparable to the existing approaches and helps reduce barriers for zebrafish laboratories studying ocular development and disease.

Structure, development, and functional morphology of the cement gland of the giant danio, Devario malabaricus

BACKGROUND

Aquatic species in several clades possess cement glands producing adhesive secretions of various strengths. In vertebrates, transient adhesive organs have been extensively studied in Xenopus laevis, other anurans, and in several fish species. However, the development of these structures is not fully understood.

RESULTS

Here, we report on the development and functional morphology of the adhesive gland of a giant danio species, Devario malabaricus. We found that the gland is localized on the larval head, is composed of goblet-like secretory cells framed by basal, bordering, and intercalated apical epithelial cells, and is innervated by the trigeminal ganglion. The gland allows nonswimming larvae to adhere to various substrates. Its secretory cells differentiate by 12 hours postfertilization and begin to disappear in the second week of life. Exogenous retinoic acid disrupts the gland's patterning. More importantly, the single mature gland emerges from fusion of two differentiated secretory cells fields; this fusion is dependent on nonmuscle myosin II function.

CONCLUSIONS

Taken together, our studies provide the first documentation of the embryonic development, structure, and function of the adhesive apparatus of a danioninae. To our knowledge, this is also the first report of a cement gland arising from convergence of two bilateral fields.

A Life in Vision

I was drawn into research in George Wald's laboratory at Harvard, where as an undergraduate and graduate student, I studied vitamin A deficiency and dark adaptation. A chance observation while an assistant professor at Harvard led to the major research of my career-to understand the functional organization of vertebrate retinas. I started with a retinal circuit analysis of the primate retina with Brian Boycott and intracellular retinal cell recordings in mudpuppies with Frank Werblin. Subsequent pharmacology studies with Berndt Ehinger primarily with fish focused on dopamine and neuromodulation. Using zebrafish, we studied retinal development, neuronal connectivity, and the effects of genetic mutations on retinal structure and function. Now semi-retired, I have returned to primate retinal circuitry, undertaking a connectomic analysis of the human fovea in Jeffrey Lichtman's laboratory.

Heart Development, Coronary Vascularization and Ventricular Maturation in a Giant Danio (Devario malabaricus)

Giant danios (genus Devario), like zebrafish, are teleosts belonging to the danioninae subfamily of cyprinids. Adult giant danios are used in a variety of investigations aimed at understanding cellular and physiological processes, including heart regeneration. Despite their importance, little is known about development and growth in giant danios, or their cardiac and coronary vessels development. To address this scarcity of knowledge, we performed a systematic study of a giant danio (Devario malabaricus), focusing on its cardiac development, from the segmentation period to ten months post-fertilization. Using light and scanning electron microscopy, we documented that its cardiovascular development and maturation proceed along well defined dynamic and conserved morphogenic patterns. The overall size and cardiovascular expansion of this species was significantly impacted by environmental parameters such as rearing densities. The coronary vasculature began to emerge in the late larval stage. More importantly, we documented two possible loci of initiation of the coronary vasculature in this species, and compared the emergence of the coronaries to that of zebrafish and gourami. This is the first comprehensive study of the cardiac growth in a Devario species, and our findings serve as an important reference for further investigations of cardiac biology using this species.

An Ultrastructural and Immunohistochemical Analysis of the Outer Plexiform Layer of the Retina of the European Silver Eel (Anguilla anguilla L)

Here we studied the ultrastructural organization of the outer retina of the European silver eel, a highly valued commercial fish species. The retina of the European eel has an organization very similar to most vertebrates. It contains both rod and cone photoreceptors. Rods are abundantly present and immunoreactive for rhodopsin. Cones are sparsely present and only show immunoreactivity for M-opsin and not for L-, S- or UV-cone opsins. As in all other vertebrate retinas, Müller cells span the width of the retina. OFF-bipolar cells express the ionotropic glutamate receptor GluR4 and ON-bipolar cells, as identified by their PKCα immunoreactivity, express the metabotropic receptor mGluR6. Both the ON- and the OFF-bipolar cell dendrites innervate the cone pedicle and rod spherule. Horizontal cells are surrounded by punctate Cx53.8 immunoreactivity indicating that the horizontal cells are strongly electrically coupled by gap-junctions. Connexin-hemichannels were found at the tips of the horizontal cell dendrites invaginating the photoreceptor synapse. Such hemichannels are implicated in the feedback pathway from horizontal cells to cones. Finally, horizontal cells are surrounded by tyrosine hydroxylase immunoreactivity, illustrating a strong dopaminergic input from interplexiform cells.

Electroretinogram analysis of the visual response in zebrafish larvae

The electroretinogram (ERG) is a noninvasive electrophysiological method for determining retinal function. Through the placement of an electrode on the surface of the cornea, electrical activity generated in response to light can be measured and used to assess the activity of retinal cells in vivo. This manuscript describes the use of the ERG to measure visual function in zebrafish. Zebrafish have long been utilized as a model for vertebrate development due to the ease of gene suppression by morpholino oligonucleotides and pharmacological manipulation. At 5-10 dpf, only cones are functional in the larval retina. Therefore, the zebrafish, unlike other animals, is a powerful model system for the study of cone visual function in vivo. This protocol uses standard anesthesia, micromanipulation and stereomicroscopy protocols that are common in laboratories that perform zebrafish research. The outlined methods make use of standard electrophysiology equipment and a low light camera to guide the placement of the recording microelectrode onto the larval cornea. Finally, we demonstrate how a commercially available ERG stimulator/recorder originally designed for use with mice can easily be adapted for use with zebrafish. ERG of larval zebrafish provides an excellent method of assaying cone visual function in animals that have been modified by morpholino oligonucleotide injection as well as newer genome engineering techniques such as Zinc Finger Nucleases (ZFNs), Transcription Activator-Like Effector Nucleases (TALENs), and Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9, all of which have greatly increased the efficiency and efficacy of gene targeting in zebrafish. In addition, we take advantage of the ability of pharmacological agents to penetrate zebrafish larvae to evaluate the molecular components that contribute to the photoresponse. This protocol outlines a setup that can be modified and used by researchers with various experimental goals.

ON-OFF Interactions in the Retina: Role of Glycine and GABA

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells and it apparently remains as signals propagate to higher brain visual centers. A fundamental question in visual neuroscience is how these two parallel pathways function: are they independent from each other or do they interact somehow? In the latter case, what kinds of mechanisms are involved and what are the consequences from this cross-talk? This review summarizes current knowledge about the types of interactions between the ON and OFF channels in nonmammalian and mammalian retina. Data concerning the ON-OFF interactions in distal retina revealed by recording of single bipolar cell activity and electroretinographic ON (b-wave) and OFF (d-wave) responses are presented. Special emphasis is put on the ON-OFF interactions in proximal retina and their dependence on the state of light adaptation in mammalian retina. The involvement of the GABAergic and glycinergic systems in the ON-OFF crosstalk is also discussed.

Possible roles of glutamate transporter EAAT5 in mouse cone depolarizing bipolar cell light responses

A remarkable feature of neuronal glutamate transporters (EAATs) is their dual functions of classical carriers and ligand-gated chloride (Cl(-)) channels. Cl(-) conductance is rapidly activated by glutamate in subtype EAAT5, which mediates light responses in depolarizing bipolar cells (DBC) in retinae of lower vertebrates. In this study, we examine whether EAAT5 also mediates the DBC light response in mouse. We took advantage of an infrared illuminated micro-injection system, and studied the effects of the EAAT blocker (TBOA) and a glutamate receptor agonist (LAP4) on the mouse electroretinogram (ERG) b-wave responses. Our results showed that TBOA and LAP4 shared similar temporal patterns of inhibition: both inhibited the ERG b-wave shortly after injection and recovered with similar time courses. TBOA inhibited the b-wave completely at mesopic light intensity with an IC50 value about 1 log unit higher than that of LAP4. The inhibitory effects of TBOA and LAP4 were found to be additive in the photopic range. Furthermore, TBOA alone inhibited the b-wave in the cone operative range in knockout mice lacking DBCRs at a low concentration that did not alter synaptic glutamate clearance activity. It also produced a stronger inhibition than that of LAP4 on the cone-driven b-wave measured with a double flash method in wildtype mice. These electrophysiological data suggest a significant role for EAAT5 in mediating cone-driven DBC light responses. Our immunohistochemistry data indicated the presence of postsynaptic EAAT5 on some DBCCs and some DBCRs, providing an anatomical basis for EAAT5's role in DBC light responses.

EML1 (CNG-modulin) controls light sensitivity in darkness and under continuous illumination in zebrafish retinal cone photoreceptors

The ligand sensitivity of cGMP-gated (CNG) ion channels in cone photoreceptors is modulated by CNG-modulin, a Ca(2+)-binding protein. We investigated the functional role of CNG-modulin in phototransduction in vivo in morpholino-mediated gene knockdown zebrafish. Through comparative genomic analysis, we identified the orthologue gene of CNG-modulin in zebrafish, eml1, an ancient gene present in the genome of all vertebrates sequenced to date. We compare the photoresponses of wild-type cones with those of cones that do not express the EML1 protein. In the absence of EML1, dark-adapted cones are ∼5.3-fold more light sensitive than wild-type cones. Previous qualitative studies in several nonmammalian species have shown that immediately after the onset of continuous illumination, cones are less light sensitive than in darkness, but sensitivity then recovers over the following 15-20 s. We characterize light sensitivity recovery in continuously illuminated wild-type zebrafish cones and demonstrate that sensitivity recovery does not occur in the absence of EML1.

The visual system of zebrafish and its use to model human ocular diseases

Free swimming zebrafish larvae depend mainly on their sense of vision to evade predation and to catch prey. Hence, there is strong selective pressure on the fast maturation of visual function and indeed the visual system already supports a number of visually driven behaviors in the newly hatched larvae.The ability to exploit the genetic and embryonic accessibility of the zebrafish in combination with a behavioral assessment of visual system function has made the zebrafish a popular model to study vision and its diseases.Here, we review the anatomy, physiology, and development of the zebrafish eye as the basis to relate the contributions of the zebrafish to our understanding of human ocular diseases.

Novel expression patterns of metabotropic glutamate receptor 6 in the zebrafish nervous system

The metabotropic glutamate receptor 6 (mGluR6 or GRM6) belongs to the class III of the metabotropic glutamate receptor family. It is the only known mGluR that mediates direct synaptic transmission in the nervous system and is thought to mediate the ON-response in the ON-pathway of the vertebrate retina. Phylogenetic and gene structure analysis indicated that the zebrafish genome harbours two mglur6 paralogs, mglur6a and mglur6b. Besides expression in the inner nuclear layer and distinct regions in the brain, both mglur6 paralogs are expressed in ganglion cells of the retina, an expression pattern which can also be observed in the downstream effector molecules gnaoa and gnaob. This unexpected expression pattern is consistent with immunohistological labeling using a peptide antibody specific for the mGluR6b paralog. These expression patterns contradict the existing view that mGluR6 is solely located on ON-bipolar cells where it functions in signal transmission. Consistent with expression in ON-bipolar cells, we report a decreased b-wave amplitude in the electroretinogram after morpholino-based downregulation of mGluR6b, showing a function in the ON response. Our data suggest more widespread functions of mGluR6 mediated signaling in the central nervous system, possibly including sign reversing synapses in the inner retina.

The giant danio (D. aequipinnatus) as a model of cardiac remodeling and regeneration

The paucity of mammalian adult cardiac myocytes (CM) proliferation following myocardial infarction (MI) and the remodeling of the necrotic tissue that ensues, result in non-regenerative repair. In contrast, zebrafish (ZF) can regenerate after an apical resection or cryoinjury of the heart. There is considerable interest in models where regeneration proceeds in the presence of necrotic tissue. We have developed and characterized a cautery injury model in the giant danio (GD), a species closely related to ZF, where necrotic tissue remains part of the ventricle, yet regeneration occurs. By light and transmission electron microscopy (TEM), we have documented four temporally overlapping processes: (1) a robust inflammatory response analogous to that observed in MI, (2) concomitant proliferation of epicardial cells leading to wound closure, (3) resorption of necrotic tissue and its replacement by granulation tissue, and (4) regeneration of the myocardial tissue driven by 5-EDU and [(3) H]thymidine incorporating CMs. In conclusion, our data suggest that the GD possesses robust repair mechanisms in the ventricle and can serve as an important model of cardiac inflammation, remodeling and regeneration.

Harmonin (Ush1c) is required in zebrafish Müller glial cells for photoreceptor synaptic development and function

Usher syndrome is the most prevalent cause of hereditary deaf-blindness, characterized by congenital sensorineural hearing impairment and progressive photoreceptor degeneration beginning in childhood or adolescence. Diagnosis and management of this disease are complex, and the molecular changes underlying sensory cell impairment remain poorly understood. Here we characterize two zebrafish models for a severe form of Usher syndrome, Usher syndrome type 1C (USH1C): one model is a mutant with a newly identified ush1c nonsense mutation, and the other is a morpholino knockdown of ush1c. Both have defects in hearing, balance and visual function from the first week of life. Histological analyses reveal specific defects in sensory cell structure that are consistent with these behavioral phenotypes and could implicate Müller glia in the retinal pathology of Usher syndrome. This study shows that visual defects associated with loss of ush1c function in zebrafish can be detected from the onset of vision, and thus could be applicable to early diagnosis for USH1C patients.

Bipolar cells in the zebrafish retina

Zebrafish are an existing model for genetic and developmental studies due to their rapid external development and transparent embryos, which allow easy manipulation and observation of early developmental stages. The application of the zebrafish model to vision research has allowed for examination of retinal development and the characteristics of different retinal cell types, including bipolar cells. In particular, bipolar cell development, including differentiation, maturation, and gene expression, has been documented, as has physiological properties, such as voltage- and ligand-gated currents, and neurotransmitter receptor and ion channel expression. Mutant strains and transgenic lines have been used to document how bipolar cell connections and/or development may be altered, and toxicological studies examining how environmental factors may impact bipolar cell activity have been performed. The purpose of this paper was to review the existing literature on zebrafish bipolar cells, to provide a comprehensive overview of current information pertaining to this retinal cell type.

Distinct retinal deficits in a zebrafish pyruvate dehydrogenase-deficient mutant

Mutations in ubiquitously expressed metabolic genes often lead to CNS-specific effects, presumably because of the high metabolic demands of neurons. However, mutations in omnipresent metabolic pathways can conceivably also result in cell type-specific effects because of cell-specific requirements for intermediate products. One such example is the zebrafish noir mutant, which we found to be mutated in the pdhb gene, coding for the E1 beta subunit of the pyruvate dehydrogenase complex. This vision mutant is described as blind and was isolated because of its vision defect-related darker appearance. A detailed morphological, behavioral, and physiological analysis of the phenotype revealed an unexpected specific effect on the retina. Surprisingly, the cholinergic amacrine cells of the inner retina are affected earlier than the photoreceptors. This might be attributable to the inability of these cells to maintain production of their neurotransmitter acetylcholine. This is reflected in an earlier loss of motion vision, followed only later by a general loss of light perception. Since both characteristics of the phenotype are attributable to a loss of acetyl-CoA production by pyruvate dehydrogenase, we used a ketogenic diet to bypass this metabolic block and could indeed partially rescue vision and prolong survival of the larvae. The noir mutant provides a case for a systemic disease with ocular manifestation with a surprising specific effect on the retina given the ubiquitous requirement for the mutated gene.

A spectral model for signal elements isolated from zebrafish photopic electroretinogram

The zebrafish photopic electroretinogram (ERG) sums isolatable elements. In each element, red-, blue-, green-, and UV- (r, g, b, and u) cone signals combine in a way that reflects retinal organization. ERG responses to monochromatic stimuli of different wavelengths and irradiances were recorded on a white rod suppressing background using superfused eyecups. Onset elements were isolated with glutamatergic blockers and response subtractions. CNQX-blocked ionotropic (AMPA/kainate) glutamate receptors; l-AP4 or CPPG-blocked metabotropic (mGluR6) glutamate receptors; TBOA-blocked glutamate transporters; and l-aspartate inactivated all glutamatergic mechanisms. Seven elements emerged: photopic PIII, the l-aspartate-isolated cone response; b1, a CNQX-sensitive early b-wave element of inner retinal origin; PII, a photopic, CNQX-insensitive composite b-wave element from ON bipolar cells; PIIm, an l-AP4/CPPG-sensitive, CNQX-insensitive, metabotropic subelement of PII; PIInm, an l-AP4/CPPG/CNQX-insensitive nonmetabotropic subelement of PII; a1nm, a TBOA-sensitive, CNQX/l-AP4/CPPG-insensitive, nonmetabotropic, postphotoreceptor a-wave element; and a2, a CNQX-sensitive a-wave element linked to OFF bipolar cells. The first five elements were fit with a spectral model that demonstrates independence of cone-color pathways. From this, Vmax and half-saturation values (k) for the contributing r-, g-, b-, and u-cone signals were calculated. Two signal patterns emerged. For PIII or PIInm, the Vmax order was Vr > Vg >> Vb approximately Vu. For b1, PII, and PIIm, the Vmax order was Vr approximately Vb > Vg > Vu. In either pattern, u-cone amplitude (Vu) was smallest, but u-cone sensitivity (ku362) was greatest, some 10-30 times greater than r cone (kr570). The spectra of b1/PII/PIIm elements peaked near b- and u-cone absorbance maxima regardless of criteria, but the spectra of PIII/PIInm elements shifted from b- toward r-cone absorbance maxima as criterion levels increased. The greatest gains in Vmax relative to PIII occurred for the b- and u-cone signals in the b1/PII/PIIm b-wave elements. This suggests a high-gain prolific metabotropic circuitry for b- and u-cone bipolar cells.

Retrograde intraflagellar transport by cytoplasmic dynein-2 is required for outer segment extension in vertebrate photoreceptors but not arrestin translocation

PURPOSE

Anterograde intraflagellar transport (IFT) is essential for photoreceptor outer segment formation and maintenance, as well as for opsin trafficking. However, the role of retrograde IFT in vertebrate photoreceptors remains unclear. The purpose of this study was to evaluate zebrafish photoreceptors lacking the retrograde IFT motor, cytoplasmic dynein-2.

METHODS

Morpholino oligonucleotides against the heavy chain (dync2-h1), light intermediate chain (dync2-li1), and intermediate chain (dync2-i1) subunits of cytoplasmic dynein-2 were injected into zebrafish embryos. Retinas and ciliated cells of these zebrafish morphants were analyzed by immunohistochemistry and transmission electron microscopy. Whole-field electroretinograms (ERGs) were performed on dynein morphants at 5 to 6 days after fertilization (dpf).

RESULTS

Zebrafish lacking cytoplasmic dynein-2 function exhibited small eyes, kidney cysts, and short photoreceptor outer segments, some of which were disorganized with accumulated vesicles. Morphant photoreceptor connecting cilia were swollen, but neither opsin nor arrestin was mislocalized, although IFT88 accumulated in the distal region of the connecting cilium. Nasal cilia were shortened and displayed cytoplasmic swelling along the axoneme. Loss of cytoplasmic dynein-2 function resulted in a significant reduction in the amplitude of ERG a-, b-, and d-waves but no change in threshold response.

CONCLUSIONS

Retrograde IFT is essential for outer segment extension and IFT protein recycling in vertebrate photoreceptors. The results show, for the first time, that the dync2-i1 subunit of cytoplasmic dynein-2 is necessary for retrograde IFT. In addition, arrestin translocation does not require retrograde IFT. Finally, the ERG results indicate that loss of cytoplasmic dynein-2 reduces the photoreceptor light response.

Ultrastructural analysis of the glutamatergic system in the outer plexiform layer of zebrafish retina

L-Glutamate, the photoreceptor neurotransmitter, depolarizes horizontal cells and OFF-bipolar cells by ionotropic receptors and hyperpolarizes ON-bipolar cells by metabotropic receptors. Despite extensive light microscopy on the distribution of glutamate receptors in zebrafish retina, there are little ultrastructural data. Given the importance of zebrafish in studies on the genetic manipulation of retinal development and function, precise data on the synaptic neurochemical organization of the zebrafish retina is needed. Immunohistochemical techniques were used to determine the ultrastructural localization of glutamate receptor subunits GluR2, GluR4, NMDA2B (NR2B) and mGluR1alpha in zebrafish outer plexiform layer (OPL). These antibodies were chosen because of an apparent conservation of localization of GluR2, GluR4 and mGluR1alpha in the vertebrate OPL, while there is some support for NMDA receptors in the OPL. GluR2-immunoreactivity (IR) was in all horizontal cell dendrites that invaginated cone pedicles and rod spherules. Three arrangements of dendrites contained GluR-IR in rod spherules: classical-type with GluR2-IR on lateral horizontal cell dendrites, a butterfly-shaped horizontal cell dendrite, and a goblet-shaped dendrite, likely of bipolar cell origin. GluR4-IR was restricted to dendrites of OFF-bipolar cells that innervated rod and cone terminals. NR2B-IR was restricted to a subtype of cone ON-bipolar cell. mGluR1alpha-IR was restricted to ON mixed rod/cone (Mb) bipolar cells whose dendrites innervated rod and cone synaptic terminals. The presence of mGluR1alpha on Mb bipolar cell dendrites is consistent with a role in retrograde endocannabinoid suppression. The subunit composition of glutamate receptors should affect the kinetics and pharmacology of these cells to glutamate receptor activation.

OFF ganglion cells cannot drive the optokinetic reflex in zebrafish

Whereas the zebrafish retina has long been an important model system for developmental and genetic studies, little is known about the responses of the inner retinal neurons. Here we report single-unit ganglion cell recordings from 5- to 6-day-old zebrafish larvae. In wild-type larvae we identify at least five subtypes of ganglion cell responses to full-field illumination, with ON-OFF and ON-type cells predominating. In the nrc mutant retina, in which the photoreceptor terminals develop abnormally, we observe normal OFF responses but abnormal ON-OFF responses and no ON responses. Previously characterized as blind, these mutants lack an optokinetic reflex (OKR), but in another behavioral assay nrc mutant fish have near-normal responses to the offset of light and slow and sluggish responses to the onset of light. Pharmacological block of the ON pathway mimics most of the nrc visual defects. We conclude that the abnormal photoreceptor terminals in nrc mutants predominantly perturb the ON pathway and that the ON pathway is necessary to drive the OKR in larval zebrafish.

Measuring the optokinetic response of zebrafish larvae

Our laboratory screens for visual mutants by examining larval eye movements in response to rotating illuminated stripes. This behavior, which is termed an optokinetic response (OKR), is a reflex that appears in zebrafish at the same time as the development of the visual system. The OKR can be accurately measured by 4 d post-fertilization, which is the age when larvae begin foraging for food. The OKR requires approximately 1 min per larva analyzed. After identifying fish with defective eye movements, we conduct secondary screens (such as histological analysis and electroretinography) to identify the subset of fish with disruptions in the function of the outer retina. This paper describes our protocol for the OKR. Our setup is simple to construct and the materials needed are inexpensive. This makes our system especially useful for new undergraduate and graduate students, as well as introductory science lecturers.

Nyctalopin is essential for synaptic transmission in the cone dominated zebrafish retina

The first synapse in the vertebrate visual system is the photoreceptor synapse between rod and cone photoreceptors and the second-order bipolar cells. Although mutations in the nyctalopin gene (NYX) in humans lead to congenital stationary night blindness (CSNB1), affecting synaptic transmission between both types of photoreceptors and ON-bipolar cells, the function of nyctalopin in cone-dominant animal models has not been studied. Because the larval zebrafish retina is cone-dominant, we isolated the zebrafish nyx ortholog and raised a polyclonal antibody against the protein. Nyctalopin is expressed postsynaptically in both synaptic layers of the retina. Functional disruption via morpholino antisense injection leads to characteristic defects in the electroretinogram and defects in visual contrast sensitivity. We therefore demonstrated that nyctalopin plays a similar role in retinal synapse function in the cone pathway as in the rod pathway, thereby creating a genetic model for CSNB1 and its effects on cone vision.

Behavioral spectral sensitivity of the zebrafish (Danio rerio)

While the zebrafish (Danio rerio) continues to become an important animal model for the investigation of the genetic and physiological bases of visual processing of the vertebrate retina, its visual behavior, particularly regarding color processing, has received little attention. The purpose of this study was to obtain behavioral spectral sensitivity functions from adult zebrafish using an appetitive instrumental conditioning procedure. A three-chamber maze was implemented to train light-adapted adult zebrafish to swim into the chamber that contained a suprathreshold monochromatic stimulus for a food reward. Visual threshold was determined by varying the stimulus irradiance using a 'two-down one-up' staircase procedure. Threshold values were obtained for wavelengths from 340 to 640 nm. Spectral sensitivity functions obtained show contributions from two nonopponent cone mechanisms (UV and S) and two opponent mechanisms (M-S and L-M). These cone mechanisms are qualitatively similar to those obtained via physiological measures from the On-responses of the zebrafish retina and optic tectum. However, the functions are not quantitatively similar suggesting that further visual processing takes place beyond the processing of the retinal circuitry and processing of the initial stages of the optic tectum. These results demonstrate that the zebrafish is an excellent model to examine and compare the relationship between physiological and behavioral color processing.

Differential expression of voltage-activated calcium currents in zebrafish retinal ganglion cells

We report a study on the characterization of voltage-activated calcium currents (I(Ca)) in retinal ganglion cells (RGCs) and the topographic distribution of RGCs that express different types of I(Ca) in zebrafish retinas. In acutely isolated zebrafish RGCs, both high-voltage-activated (HVA; peak activation potential +7.4 +/- 1.1 mV) and low-voltage-activated (LVA; peak activation potential -33.0 +/- 1.2 mV) I(Ca) were recorded. HVA I(Ca) were recorded in all of the tested RGCs, whereas LVA I(Ca) were recorded in approximately one-third of the tested cells. In RGCs that expressed both HVA and LVA I(Ca), the two currents were readily separated by depolarizing the cell membrane to different voltages from different holding potentials. Among RGCs that expressed LVA I(Ca), some cells expressed large LVA I(Ca) (up to 130 pA), whereas others expressed small LVA I(Ca) (approximately 20 pA). RGCs that expressed large and small LVA I(Ca) were designated as class I and class II cells, respectively, and RGCs that expressed only HVA I(Ca) were designated as class III cells. The topographic distribution of cell classes was similar in various areas of the retina. In the nasal-ventral retina, for example, class III cells outnumbered class I and class II cells by 10.8- and 2.6-fold, respectively. In the temporal and dorsal retinas, the density of class III cells slightly decreased, whereas the density of class I and class II cells increased. The differential expression of I(Ca) in RGCs may correlate with the development and function of the retina.

Dopamine modulates voltage-activated potassium currents in zebrafish retinal on bipolar cells

We report a study of the characterization of voltage-activated potassium (K+) currents in retinal ON bipolar cells in zebrafish. At single-channels levels, the open probability of the K+ channels increased when the membrane potential was increased. The maximal open proportion was 0.76+/-0.05 under our testing conditions. In whole-cell recordings, the K+ current displayed two exponential components with the activation time constants of 11-22 msec (tau1) and 0.8-4 msec (tau2). Dopamine modulated the K+ current. Dopamine reduced the time constant tau2 when the membrane potential was depolarized to high voltages. A decrease in K+ current was seen when dopamine D1 receptors were selectively activated by SKF38393 or when the D1 receptor-coupled G-proteins were activated by GTP-gamma-S. The activation of adenylate cyclase by forskolin or the increase of intracellular cAMP concentrations by 8-Br-cAMP or Sp-cAMPS also resulted in a decrease in K+ current. Together, the data suggest that dopamine modulates the K+ current via D1 receptor-coupled G-protein pathways.

Night blindness and abnormal cone electroretinogram ON responses in patients with mutations in the GRM6 gene encoding mGluR6

We report three unrelated patients with mutations in the GRM6 gene that normally encodes the glutamate receptor mGluR6. This neurotransmitter receptor has been shown previously to be present only in the synapses of the ON bipolar cell dendrites, and it mediates synaptic transmission from rod and cone photoreceptors to this type of second-order neuron. Despite the synaptic defect, best visual acuities were normal or only moderately reduced (20/15 to 20/40). The patients were night blind from an early age, and when maximally dark-adapted, they could perceive lights only with an intensity equal to or slightly dimmer than that normally detected by the cone system (i.e., 2-3 log units above normal). Electroretinograms (ERGs) in response to single brief flashes of light had clearly detectable a-waves, which are derived from photoreceptors, and greatly reduced b-waves, which are derived from the second-order inner retinal neurons. ERGs in response to sawtooth flickering light indicated a markedly reduced ON response and a nearly normal OFF response. There was no subjective delay in the perception of suddenly appearing white vs. black objects on a gray background. These patients exemplify a previously unrecognized, autosomal recessive form of congenital night blindness associated with a negative ERG waveform.

Retinal bipolar cell input mechanisms in giant danio. III. ON-OFF bipolar cells and their color-opponent mechanisms

Whole cell patch recording was performed from morphologically identified cone-driven on-off bipolar cells (Cabs) in giant danio retinal slices to study their glutamate receptors and light-evoked responses. Specific agonists were puffed in the presence of cobalt, picrotoxin, and strychnine to identify glutamate receptors on these cells. Most Cabs responded to both the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor agonist kainate and the excitatory amino acid transporter (EAAT) substrate D-aspartate, and both responses were localized to the dendrites. Kainate generated depolarizations whereas D-aspartate had E(rev) close to E(Cl) and generated hyperpolarizations, indicating that the AMPA/kainate receptors are sign-preserving, whereas the EAATs are sign-inverting. In response to white light, some Cabs gave on bipolar cell-like responses whereas others gave off bipolar cell-like ones, but many cells' responses had both on and off bipolar cell components. In response to appropriately colored center-selective stimuli, many Cabs responded to short and long wavelengths with opposite polarities and were thus double color-opponent. The depolarizing components of the responses to white or colored stimuli were suppressed by the EAAT blocker DL-threo-beta-benzyloxyaspartate (TBOA), whereas the hyperpolarizing components were reduced by the AMPA/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). These results are consistent with the hypothesis that both EAATs and AMPA/kainate receptors are involved in the generation of light-evoked responses in Cabs and that they confer these cells with on and off bipolar cell properties, respectively. Cabs can generate double color-opponent center responses by receiving inputs from certain cones through EAATs and from other cones through AMPA/kainate receptors.