Molecular mechanisms of vertebrate retina development: Implications for ganglion cell and photoreceptor patterning

Toxicity of triphenyltin on the development of retinal axons in zebrafish at low dose

The impacts of triphenyltin (TPT) on ecological health are of particular concern due to the unexpectedly high levels found in wild fish around the world. Here, zebrafish embryos were exposed to TPT via in ovo nano-injection to study its toxicity on the development of retinal axons in fish. Lipophilic dye labeling revealed obvious defects in retinal axon development in larvae with normally shaped eyes, with incidences of 0, 1.08%, 2.66%, 4.26%, and 6.85% observed in the control, 0.8, 4.0, 20.0, and 100ng TPT-Cl/g wet weight (ww) exposure groups, respectively, showing a dose-dependent increase. Since the lowest observable effective concentration of TPT to induce retinal axon development defects was 0.8ng TPT-Cl/g ww, which is lower than the concentrations in wild fish eggs, this defect would occur in wild fish larvae. Alterations in the expressions of pax6 and ephrinBs, which regulate the establishment of retinal polarity, were correlated with defect incidence. Expression levels of the CYP26A1 gene and protein were significantly up-regulated in all exposure groups compared with the control, which may lead to significant decreases in concentrations of all-trans retinoic acid (atRA). Such a disruption of RA metabolism would, at least partly, contribute to the incidence of developmental defects in retinal axons. This study is the first to report that TPT can interfere with development of retinal axons in fish at low dose.

Molecular Heterogeneity Within the Clinical Diagnosis of Pericentral Retinal Degeneration

PURPOSE

To characterize in detail the phenotype and genotype of patients with pericentral retinal degeneration (PRD).

METHODS

Patients were screened for an annular ring scotoma ranging from 3° to 40° (n = 28, ages 24-71) with kinetic perimetry. All patients had pigmentary retinopathy in the region of the dysfunction. Further studies included cross-sectional and en face imaging, static chromatic perimetry, and electroretinography. Molecular screening was performed.

RESULTS

Genotypes of 14 of 28 PRD patients were identified: There were mutations in eight different genes previously associated with autosomal dominant or autosomal recessive RDs. Kinetic fields monitored in some patients over years to more than a decade could be stable or show increased extent of the scotoma. Electroretinograms were recordable but with different severities of dysfunction. Patterns of photoreceptor outer nuclear layer (ONL) loss corresponded to the distribution of visual dysfunction. Outer nuclear layer thickness topography and en face imaging indicated that the greatest disease expression was in the area of known highest rod photoreceptor density.

CONCLUSIONS

Molecular heterogeneity was a feature of the PRD phenotype. Many of the molecular causes were also associated with other phenotypes, such as maculopathies, typical retinitis pigmentosa (RP) and cone-rod dystrophy. The pericentral pattern of retinal degeneration is thus confirmed to be an uncommon phenotype of many different genotypes rather than a distinct disease entity.

Opposing Shh and Fgf signals initiate nasotemporal patterning of the zebrafish retina

The earliest known determinants of retinal nasotemporal identity are the transcriptional regulators Foxg1, which is expressed in the prospective nasal optic vesicle, and Foxd1, which is expressed in the prospective temporal optic vesicle. Previous work has shown that, in zebrafish, Fgf signals from the dorsal forebrain and olfactory primordia are required to specify nasal identity in the dorsal, prospective nasal, optic vesicle. Here, we show that Hh signalling from the ventral forebrain is required for specification of temporal identity in the ventral optic vesicle and is sufficient to induce temporal character when activated in the prospective nasal retina. Consequently, the evaginating optic vesicles become partitioned into prospective nasal and temporal domains by the opposing actions of Fgfs and Shh emanating from dorsal and ventral domains of the forebrain primordium. In absence of Fgf activity, foxd1 expression is established irrespective of levels of Hh signalling, indicating that the role of Shh in promoting foxd1 expression is only required in the presence of Fgf activity. Once the spatially complementary expression of foxd1 and foxg1 is established, the boundary between expression domains is maintained by mutual repression between Foxd1 and Foxg1.

Summary: In the fish eye, Hh signalling from the ventral forebrain regulates spatial identity in the retina by promoting foxd1 expression. This role is required only in the presence of Fgf activity.

Development of dendritic form and function

The nervous system is populated by numerous types of neurons, each bearing a dendritic arbor with a characteristic morphology. These type-specific features influence many aspects of a neuron's function, including the number and identity of presynaptic inputs and how inputs are integrated to determine firing properties. Here, we review the mechanisms that regulate the construction of cell type-specific dendrite patterns during development. We focus on four aspects of dendrite patterning that are particularly important in determining the function of the mature neuron: (a) dendrite shape, including branching pattern and geometry of the arbor; (b) dendritic arbor size;

A critical period for omega-3 nutritional supplementation in the development of the rodent visual system

Retinocollicular connections form precise topographical maps that are normally completed through the selective elimination of misplaced axons and the stabilization of topographically ordered axon terminals during early development. Omega-3 fatty acids, acquired exclusively through the diet, and its main metabolite, docosahexaenoic acid (DHA), are involved in brain development and synaptic maturation. We have previously shown that the nutritional restriction of omega-3/DHA results in abnormal retinocollicular topographical fine-tuning. Therefore, we studied the role of omega-3 fatty acids nutritional supplementation and the developmental time windows during which this postnatal supplementation would restore normal topographical maps in the visual system. Female rats and their litters were chronically fed with either control (soy oil) or restricted omega-3 (coconut oil) diets. Fish oil supplementation was introduced between either postnatal day (PND) 7-13, PND7-28 or PND21-42. At PND13, PND28 or PND42, animals received an anterograde eye injection of a neuronal tracer to visualize retinocollicular axons. Confirming previous observations we found that an omega-3/DHA deficiency resulted in an abnormally high innervation density of retinal axons at the visual layers of the superior colliculus (SC). Although a short-term fish oil supplementation between PND7-13 could not restore normal retinocollicular topography, an extended treatment between PND7-28 completely recovered normal innervation densities of retinotectal axons. However, a late onset supplementation protocol, between PND28-42, was no longer effective in the restoration of the abnormal topographical pattern induced by an early omega-3 nutritional malnutrition. The results suggest a critical period for omega3/DHA dietary intake for the proper development of visual topographical maps.

Early divergence of central and peripheral neural retina precursors during vertebrate eye development

BACKGROUND

During development of the vertebrate eye, optic tissue is progressively compartmentalized into functionally distinct tissues. From the central to the peripheral optic cup, the original optic neuroepithelial tissue compartmentalizes, forming retina, ciliary body, and iris. The retina can be further sub-divided into peripheral and central compartments, where the central domain is specialized for higher visual acuity, having a higher ratio and density of cone photoreceptors in most species.

RESULTS

Classically, models depict a segregation of the early optic cup into only two domains, neural and non-neural. Recent studies, however, uncovered discrete precursors for central and peripheral retina in the optic vesicle, indicating that the neural retina cannot be considered as a single unit with homogeneous specification and development. Instead, central and peripheral retina may be subject to distinct developmental pathways that underlie their specialization.

CONCLUSIONS

This review focuses on lineage relationships in the retina and revisits the historical context for segregation of central and peripheral retina precursors before overt eye morphogenesis.

ADAM10 mediates N-cadherin ectodomain shedding during retinal ganglion cell differentiation in primary cultured retinal cells from the developing chick retina

Here, we examined the role of ADAM10 during retinal cell differentiation in retinal sections and in vitro cultures of developing chick retinal cells from embryonic day 6 (ED6). Immunohistochemistry showed that ADAM10 is abundantly expressed in the inner zone of neuroblastic layer at ED5, and it becomes more highly expressed in the ganglion cell layer at ED7 and ED9. Western blotting confirmed that ADAM10 was expressed as an inactive pro-form that was processed to a shorter, active form in control cultured cells, but in cultures treated with an ADAM10 inhibitor (GI254023X) and ADAM10-specific siRNA, the level of mature ADAM10 decreased. Phase-contrast microscopy showed that long neurite extensions were present in untreated cultures 24 h after plating, whereas cultures treated with GI254023X showed significant decreases in neurite extension. Immunofluorescence staining revealed that there were far fewer differentiated ganglion cells in ADAM10 siRNA and GI254023X-treated cultures compared to controls, whereas the photoreceptor cells were unaltered. The Pax6 protein was more strongly detected in the differentiated ganglion cells of control cultures compared to ADAM10 siRNA and GI254023X-treated cultures. N-cadherin ectodomain shedding was apparent in control cultures after 24 h, when ganglion cell differentiation was observed, but ADAM10 siRNA and GI254023X treatment inhibited these processes. In contrast, N-cadherin staining was strongly detected in photoreceptor cells regardless of ADAM10 siRNA and GI254023X treatment. Taken together, these data indicate that the inhibition of ADAM10 can inhibit Pax6 expression and N-cadherin ectodomain shedding in retinal cells, possibly affecting neurite outgrowth and ganglion cell differentiation.

Temporal and spatial expression of CCN3 during retina development

NOV/CCN3 is one of the founding members of the CCN (Cyr61 CTGF NOV) family. In the avian retina, CCN3 expression is mostly located within the central region of the inner nuclear layer. As retinal development progresses and this retinal layer differentiates and matures, CCN3 expression forms a dorsal-ventral and a central-peripheral gradient. CCN3 is produced by two glial cell types, peripapillary cells and Müller cells, as well as by horizontal, amacrine, and bipolar interneurons. In retinal neurons and Müller cell cultures, CCN3 expression is induced by activated BMP signaling, whereas Notch signaling decreases CCN3 mRNA and protein levels in Müller cells and has no effect in retinal neurons. In Müller cells, the CCN3 expression detected may thus result from a balance between the Notch and BMP signaling pathways.

Nutritional restriction of omega-3 fatty acids alters topographical fine tuning and leads to a delay in the critical period in the rodent visual system

The development and maturation of sensory systems depends on the correct pattern of connections which occurs during a critical period when axonal elimination and synaptic plasticity are involved in the formation of topographical maps. Among the mechanisms involved in synaptic stabilization, essential fatty acids (EFAs), available only through diet, appear as precursors of signaling molecules involved in modulation of gene expression and neurotransmitter release. Omega-3 fatty acids, such as docosahexaenoic acid (DHA), are considered EFAs and are accumulated in the brain during fetal period and neonatal development. In this study, we demonstrated the effect of omega-3/DHA nutritional restriction in the long-term stabilization of connections in the visual system. Female rats were fed 5 weeks before mating with either a control (soy oil) or a restricted (coconut oil) diet. Litters were fed until postnatal day 13 (PND13), PND28 or PND42 with the same diets when they received an intraocular injection of HRP. Another group received a single retinal lesion at the temporal periphery at PND21. Omega-3 restriction induced an increase in the optical density in the superficial layers of the SC, as a result of axonal sprouting outside the main terminal zones. This effect was observed throughout the SGS, including the ventral and intermediate sub-layers at PND13 and also at PND28 and PND42. The quantification of optical densities strongly suggests a delay in axonal elimination in the omega3(-) groups. The supplementation with fish oil (DHA) was able to completely reverse the abnormal expansion of the retinocollicular projection. The same pattern of expanded terminal fields was also observed in the ipsilateral retinogeniculate pathway. The critical period window was studied in lesion experiments in either control or omega-3/DHA restricted groups. DHA restriction induced an increased sprouting of intact, ipsilateral axons at the deafferented region of the superior colliculus compared to the control group, revealing an abnormal extension of the critical period. Finally, in omega-3 restricted group we observed in the collicular visual layers normal levels of GAP-43 with decreased levels of its phosphorylated form, p-GAP-43, consistent with a reduction in synaptic stabilization. The data indicate, therefore, that chronic dietary restriction of omega-3 results in a reduction in DHA levels which delays axonal elimination and critical period closure, interfering with the maintenance of terminal fields in the visual system.

Cell biological regulation of division fate in vertebrate neuroepithelial cells

The developing nervous system derives from neuroepithelial progenitor cells that divide to generate all of the mature neuronal types. For the proper complement of cell types to form, the progenitors must produce postmitotic cells, yet also replenish the progenitor pool. Progenitor divisions can be classified into three general types: symmetric proliferative (producing two progenitors), asymmetric neurogenic (producing one progenitor and one postmitotic cell), and symmetric neurogenic (producing two postmitotic cells). The appropriate ratios for these modes of cell division require intrinsic polarity, which is one of the characteristics that define neuroepithelial progenitor cells. The type of division an individual progenitor undergoes can be influenced by cellular features, or behaviors, which are heterogeneous within the population of progenitors. Here we review three key cellular parameters, asymmetric inheritance, cell cycle kinetics, and interkinetic nuclear migration, and the possible mechanisms for how these features influence progenitor fates.

Expression and light sensitivity of clock genes Per1 and Per2 and immediate-early gene c-fos within the retina of early postnatal Wistar rats

Mammalian retina contains a circadian clock that is composed of components similar to those of the master circadian clock within the suprachiasmatic nuclei of the hypothalamus. The aim of the present study was to elucidate whether, when, and where the transcripts of the clock genes Per1 and Per2 and the immediate early gene c-fos are spontaneously expressed and/or induced by light in the newborn rat retina. At postnatal day 1 (P1), P3, P5, and P10, Wistar rat pups were released into constant darkness, and a 30-minute light pulse was administered during the subjective day or during the first or second part of subjective night. Gene expression was determined 30 minutes, 1 hour, 2 hours, and 4 hours after the light pulse by in situ hybridization followed by emulsion autoradiography. Endogenous expression of Per1 was detected in the neuroblastic retina, and Per2 expression was detected in the inner part of the neuroblastic retina from birth. Light pulses induced c-fos expression in ganglion cells from P1. Until P5, the cells were localized in the dorsal part of the retina, but, at P10, they were already distributed across the entire retinal circumference. Light pulses also induced the expression of c-fos and Per1 in the retinal pigment epithelium until P3, but not afterward. Expression of the Per2 gene was not photoresponsive until P10. These data demonstrate that the rat retina is light-sensitive immediately after birth. During early postnatal development, the spatial distribution of spontaneous and light-induced gene expression within the retinal layers changes gradually.

Retinal development and function in a 'blind' mole

Animals adapted to dark ecotopes may experience selective pressure for retinal reduction. No previous studies have explicitly addressed the molecular basis of retinal development in any fossorial mammal. We studied retinal development and function in the Iberian mole Talpa occidentalis, which was presumed to be blind because of its permanently closed eyes. Prenatal retina development was relatively normal, with specification of all cell types and evidence of dorsoventral regionalization. Severe developmental defects occurred after birth, subsequent to lens abnormalities. 'Blind' Iberian moles had rods, cones and rod nuclear ultrastructure typical of diurnal mammals. DiI staining revealed only contralateral projections through the optic chiasm. Y-maze experiments demonstrated that moles retain a photoavoidance response. Over-representation of melanopsin-positive retinal ganglion cells that mediate photoperiodicity was observed. Hence, molecular pathways of eye development in Iberian moles retain the adaptive function of rod/cone primary vision and photoperiodicity, with no evidence that moles are likely to completely lose their eyes on an evolutionary time scale.

Progenitor cells of the rod-free area centralis originate in the anterior dorsal optic vesicle

Background

Nervous system development is dependent on early regional specification to create functionally distinct tissues within an initially undifferentiated zone. Within the retina, photoreceptors are topographically organized with rod free area centrales faithfully generated at the centre of gaze. How does the developing eye regulate this placement? Conventional wisdom indicates that the distal tip of the growing optic vesicle (OV) gives rise to the area centralis/fovea. Ectopic expression and ablation studies do not fully support this view, creating a controversy as to the origin of this region. In this study, the lineage of cells in the chicken OV was traced using DiI. The location of labelled cells was mapped onto the retina in relation to the rod-free zone at embryonic (E) 7 and E17.5. The ability to regenerate a rod free area after OV ablation was determined in conjunction with lineage tracing.

Results

Anterior OV gave rise to cells in nasal retina and posterior OV became temporal retina. The OV distal tip gave rise to cells above the optic nerve head. A dorsal and anterior region of the OV correlated with cells in the developing rod free area centralis. Only ablations including the dorsal anterior region gave rise to a retina lacking a rod free zone. DiI application after ablation indicated that cells movements were greater along the anterior/posterior axis compared with the dorsal/ventral axis.

Conclusion

Our data support the idea that the chicken rod free area centralis originates from cells located near, but not at the distal tip of the developing OV. Therefore, the hypothesis that the area centralis is derived from cells at the distal tip of the OV is not supported; rather, a region anterior and dorsal to the distal tip gives rise to the rod free region. When compared with other studies of retinal development, our results are supported on molecular, morphological and functional levels. Our data will lead to a better understanding of the mechanisms underlying the topographic organization of the retina, the origin of the rod free zone, and the general issue of compartmentalization of neural tissue before any indication of morphological differentiation.

Labelling and targeted ablation of specific bipolar cell types in the zebrafish retina

Background

Development of a functional retina depends on regulated differentiation of several types of neurons and generation of a highly complex network between the different types of neurons. In addition, each type of retinal neuron includes several distinct morphological types. Very little is known about the mechanisms responsible for generating this diversity of retinal neurons, which may also display specific patterns of regional distribution.

Results

In a screen in zebrafish, using a trapping vector carrying an engineered yeast Gal4 transcription activator and a UAS:eGFP reporter cassette, we have identified two transgenic lines of zebrafish co-expressing eGFP and Gal4 in specific subsets of retinal bipolar cells. The eGFP-labelling facilitated analysis of axon terminals within the inner plexiform layer of the adult retina and showed that the fluorescent bipolar cells correspond to previously defined morphological types. Strong regional restriction of eGFP-positive bipolar cells to the central part of the retina surrounding the optic nerve was observed in adult zebrafish. Furthermore, we achieved specific ablation of the labelled bipolar cells in 5 days old larvae, using a bacterial nitroreductase gene under Gal4-UAS control in combination with the prodrug metronidazole. Following prodrug treatment, nitroreductase expressing bipolar cells were efficiently ablated without affecting surrounding retina architecture, and recovery occurred within a few days due to increased generation of new bipolar cells.

Conclusion

This report shows that enhancer trapping can be applied to label distinct morphological types of bipolar cells in the zebrafish retina. The genetic labelling of these cells yielded co-expression of a modified Gal4 transcription activator and the fluorescent marker eGFP. Our work also demonstrates the potential utility of the Gal4-UAS system for induction of other transgenes, including a bacterial nitroreductase fusion gene, which can facilitate analysis of bipolar cell differentiation and how the retina recovers from specific ablation of these cells.

Intrinsic patterning and experience-dependent mechanisms that generate eye-specific projections and binocular circuits in the visual pathway

A defining feature of the mammalian nervous system is its complex yet precise circuitry. The mechanisms which underlie the generation of neural connectivity are the topic of intense study in developmental neuroscience. The mammalian visual pathway demonstrates precise retinotopic organization in subcortical and cortical pathways, together with the alignment and matching of eye-specific projections, and sophisticated cortical circuitry that enables the extraction of features underlying vision. New approaches employing molecular-genetic analyses, transgenic mice, novel recombinant probes, and high-resolution imaging are contributing to rapid progress and a new synthesis in the field. These approaches are revealing the ways in which intrinsic patterning mechanisms act in concert with experience-dependent mechanisms to shape visual projections and circuits.