Survival of ganglion cells which form the retino-retinal projection during optic nerve regeneration in the frog

Retinal ganglion cell interactions shape the developing mammalian visual system

Retinal ganglion cells (RGCs) serve as a crucial communication channel from the retina to the brain. In the adult, these cells receive input from defined sets of presynaptic partners and communicate with postsynaptic brain regions to convey features of the visual scene. However, in the developing visual system, RGC interactions extend beyond their synaptic partners such that they guide development before the onset of vision. In this Review, we summarize our current understanding of how interactions between RGCs and their environment influence cellular targeting, migration and circuit maturation during visual system development. We describe the roles of RGC subclasses in shaping unique developmental responses within the retina and at central targets. Finally, we highlight the utility of RNA sequencing and genetic tools in uncovering RGC type-specific roles during the development of the visual system.

A Retino-retinal Projection Guided by Unc5c Emerged in Species with Retinal Waves

The existence of axons extending from one retina to the other has been reported during perinatal development in different vertebrates. However, it has been thought that these axons are either a labeling artifact or misprojections. Here, we show unequivocally that a small subset of retinal ganglion cells (RGCs) project to the opposite retina and that the guidance receptor Unc5c, expressed in the retinal region where the retinal-retinal (R-R) RGCs are located, is necessary and sufficient to guide axons to the opposite retina. In addition, Netrin1, an Unc5c ligand, is expressed in the ventral diencephalon in a pattern that is consistent with impeding the growth of Unc5c-positive retinal axons into the brain. We also have generated a mathematical model to explore the formation of retinotopic maps in the presence and absence of a functional connection between both eyes. This model predicts that an R-R connection is required for the bilateral coordination of axonal refinement in species where refinement depends upon spontaneous retinal waves. Consistent with this idea, the retinal expression of Unc5c correlates with the existence and size of an R-R projection in different species and with the extent of axonal refinement in visual targets. These findings demonstrate that active guidance drives the formation of the R-R projection and suggest an important role for these projections in visual mapping to ensure congruent bilateral refinement.

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A subset of retinal ganglion cells project to the contralateral retina

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Unc5c mediates the formation of the retina-retina projection

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Unc5c retinal expression correlates with extent of refinement in visual targets

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Congruency of visual maps in species with retinal waves may rely on R-R axons

Retinal cross talk in the mammalian visual system

The existence and functional relevance of efferent optic nerve fibers in mammals have long been debated. While anatomical evidence for cortico-retinal and retino-retinal projections is substantial, physiological evidence is lacking, as efferent fibers are few in number and are severed in studies of excised retinal tissue. Here we show that interocular connections contribute to retinal bioelectrical activity in adult mammals. Full-field flash electroretinograms (ERGs) were recorded from one or both eyes of Brown-Norway rats under dark-adapted (n = 16) and light-adapted (n = 11) conditions. Flashes were confined to each eye by an opaque tube that blocked stray light. Monocular flashes evoked a small (5-15 μV) signal in the nonilluminated eye, which was named "crossed ERG" (xERG). The xERG began under dark-adapted conditions with a positive (xP1) wave that peaked at 70-90 ms and ended with slower negative (xN1) and positive (xP2) waves from 200 to 400 ms. xN1 was absent under light-adapted conditions. Injection of tetrodotoxin in either eye (n = 15) eliminated the xERG. Intraocular pressure elevation of the illuminated eye (n = 6) had the same effect. The treatments also altered the ERG b-wave in both eyes, and the alterations correlated with xERG disappearance. Optic nerve stimulation (n = 3) elicited a biphasic compound action potential in the nonstimulated nerve with 10- to 13-ms latency, implying that the xERG comes from slow-conducting (W type) fibers. Monocular dye application (n = 7) confirmed the presence of retino-retinal ganglion cells in adult rats. We conclude that mammalian eyes communicate directly with each other via a handful of optic nerve fibers. The cross talk alters retinal activity in rats, and perhaps other animals.

Retino-retinal projection in juvenile and young adult rats and mice

Identification of retino-retinal projecting RGCs (ret-ret RGCs) has been accomplished by tracing RGCs in one retina after intravitreal injection of different tracers in the other eye. In mammals, rabbit and rat, ret-ret RGCs are scarce and more abundant in newborn than in adult animals. To our knowledge, ret-ret RGCs have not been studied in mice. Here we purpose to revisit the presence of ret-ret RGCs in juvenile and young adult rats and mice by using retrograde tracers applied to the contralateral optic nerve instead of intravitreally. In P20 (juvenile) and P60 (young adult) animals, the left optic nerve was intraorbitally transected and Fluorogold (rats) or its analogue OHSt (mice) were applied onto its distal stump. P20 animals were sacrificed 3 (mice) or 5 (rats) days later and adult animals at 5 (mice) or 7 (rats) days. Right retinas were dissected as flat-mounts and double immunodetected for Brn3a and melanopsin. Ret-ret RGCs were those with tracer accumulation in their somas. Out of them some expressed Brn3a and/or melanopsin, while other were negative for both markers. In young adult rats, we found 2 ret-ret RGCs displaced to the inner nuclear layer. In both species, ret-ret RGCs are quite scarce and found predominantly in the nasal retina. In juvenile animals there are significantly more ret-ret RGCs (9 ± 3, rats, 13 ± 3 mice) than in young adult ones (5 ± 6 rats, 7 ± 3 mice). Finally, juvenile and young adult mice have more ret-ret RGCs than rats.

The retino-retinal projection: Tracing retinal ganglion cells projecting to the contralateral retina

We investigated the presence of a direct retino-retinal (R-R) projection between the two eyes via the optic chiasm of retinal ganglion cells (RGCs) in adult Long-Evans rats. We also explored the presence of collateral projections originating from these cells to the brain. In the first group of animals, right optic nerves (ONs) were orbitally transected approximately 2mm behind the globe followed by application of fluorochrome (2% Fluorogold [FG]) to the optic nerve stump to retrogradely label the R-R projection RGCs (R-RGCs) on the contralateral side. Animals were then sacrificed after 3, 5, 7, or 21 days. Contralateral retinas were fixed, whole-mounted, and imaged for R-RGCs. In a second group of animals, RGCs were retrogradely labeled with 15% rhodamine-β-isothiocynate (RITC) at the superior colliculi, where approximately 96% of rat RGCs synapse. Seven days later, the right ONs were transected and 2% FG applied to the proximal and distal ON stumps. Animals were then sacrificed after 5 days. Contralateral retinas were examined for co-labeled (RITC/FG) RGCs. Control rats underwent the same procedures excluding fluorescent tracer application. In the first group of animals, the number of R-RGCs in the contralateral eye ranged from 3 to 25 and did not depend on survival time. The second group of animals revealed evidence of co-labeled contralateral RGCs. Results suggest that a greater number of R-RGCs persist into adulthood than previously reported [M. Müller, H. Holländer, 1988]. Furthermore, the presence of co-labeled RGCs in the contralateral eye indicates that in adult rodents some R-R projections have a collateral projection to the brain, whereas previous reports had only found collateral projections in newborns.

Failure to restore vision after optic nerve regeneration in reptiles: Interspecies variation in response to axotomy

Optic nerve regeneration within the reptiles is variable. In a snake, Viper aspis, and the lizard Gallotia galloti, regeneration is slow, although some retinal ganglion cell (RGC) axons eventually reach the visual centers (Rio et al. [1989] Brain Res 479:151-156; Lang et al. [1998] Glia 23:61-74). By contrast, in a lizard, Ctenophorus ornatus, numerous RGC axons regenerate rapidly to the visual centers, but unless animals are stimulated visually, the regenerated projection lacks topography and animals remain blind via the experimental eye (Beazley et al. [2003] J. Neurotrauma 20:1263-1269). V. aspis, G. galloti, and C. ornatus belong respectively to the Serpentes, Lacertidae, and Agamidae within the Eureptilia, the major modern group of living reptiles comprising the Squamata (snakes, lizards, and geckos) and the Crocodyllia. Here we have extended the findings on Eureptilia to include two geckos (Gekkonidae), Cehyra variegata and Nephrurus stellatus. We also examined a turtle, Chelodina oblonga, the Testudines being the sole surviving representatives of the Parareptilia, the more ancient reptilian group. In all three species, visually elicited behavioral responses were absent throughout regeneration, a result supported electrophysiologically; axonal tracing revealed that only a small proportion of RGC axons crossed the lesion and none entered the contralateral optic tract. RGC axons failed to reach the chiasm in C. oblonga, and in G. variegata, and N. stellatus RGC axons entered the opposite optic nerve; a limited ipsilateral projection was seen in G. variegata. Our results support a heterogeneous response to axotomy within the reptiles, each of which is nevertheless dysfunctional.

Development of visual projections follows an avian/mammalian-like sequence in the lizard Ctenophorus ornatus

Development of primary visual projections was examined in a lizard Ctenophorus ornatus by anterograde and retrograde tracing with DiI and by GAP-43 immunohistochemistry. Visual pathway development was essentially similar to that in birds and mammals and thus differed from patterns in fish or amphibians. A number of features characterised the development as mammalian-like. Three phases occurred in rapid succession after laying: outgrowth (2-3 weeks, early), exuberance (4-5 weeks, intermediate), and retraction to the adult pattern (6-8 weeks, late) at about the time of hatching and eye opening. Furthermore, ipsilateral projections developed with only a slight lag relative to the contralateral ones. The dorsally located fovea could be identified from early stages. Optic axons formed transient exuberant projections to the ipsilateral optic tectum, to the opposite optic nerve, and to nonvisual regions. The pattern resembled that formed in the long term by regenerating optic axons in C. ornatus (Dunlop et al. [2000b] J. Comp. Neurol. 416:188-200), suggesting that axons recognise molecular signals associated with the initial exuberant innervation but not those associated with subsequent refinement.

Extent of retinal ganglion cell death in the frog Litoria moorei after optic nerve regeneration induced by lesions of different sizes

Some amphibian retinal ganglion cells die during optic nerve regeneration. Here we have investigated whether ganglion cell death in the frog Litoria moorei is associated with the lesion site. For one experimental series, the optic nerve lesion extended for 0.15 mm; in the other, it extended for 1.5 mm. The extent of ganglion cell death was estimated from cresyl violet-stained whole mounts at 24 weeks post lesion. In other animals, individual regenerating axons were visualised in the optic nerve by horseradish peroxidase (HRP) labelling from 1 day to 24 weeks post lesion; counterstaining with cresyl violet allowed examination of cells that repopulated the lesion site. Ganglion cell numbers fell significantly more after an extensive than after a localised lesion, long-term losses being 50% and 34%, respectively (P < 0.05). Regenerating axons were delayed in their passage across the cell-poor extensive lesion compared with the relatively cell-rich localised lesion. The differing rates of regeneration between series were matched by greater delay after extensive lesion in the return of visually guided behaviour as assessed by optokinetic horizontal head nystagmus. We suggest that delays in regeneration after an extensive lesion exacerbate ganglion cell death, indicating that conditions within the lesion are associated with the death of some ganglion cells.

Genesis, neurotrophin responsiveness, and apoptosis of a pronounced direct connection between the two eyes of the chick embryo: a natural error or a meaningful developmental event?

Unilateral intraocular injections of either of two fluorescent carbocyanine dyes into the embryonic chick eye resulted in both retrograde staining of ganglion cells (GCs) in the eye contralateral to site of injection and anterograde labeling of axons whose cell bodies were located within the injected eye. This prominent retino-retinal projection formed by thousands of GCs having a nasal origin and temporal termination appeared at embryonic day 6 (E6), attained its maximum intensity at E13-E14, and gradually disappeared until E18. The axonal growth cones ended superficially and never penetrated deeper layers of the retina. Treatment of the projection with BDNF resulted in massive terminal branching of the axons within deeper layers of the target retina. Double injection into the eye and the isthmo-optic nucleus showed a concomitant ingrowth of axons in the contralateral retina. Individual GCs died between E9 and E13, but massive apoptotic cell death was mainly monitored at E14 and later. Disintegrated cells showed typical images of apoptosis. Because degenerating cells were prelabeled with the membranophilic fluorescent carbocyanine dye, their death allowed the concomitant visualization of phagocytosing cells, too. Radial Müller glia were the only class of cells observed to become phagocytotic between E9 and E16. These cells became replaced exclusively with microglial cells from E17 on. The results suggest that the topologically restricted retino-retinal projection may have some developmental significance rather than representing a massive erroneous projection. Most likely, the projection may serve as a "template" to guide centrifugal isthmo-optic axons into the retina.

Impaired vision for binocular tasks after unilateral optic nerve regeneration in the frog Litoria moorei

Behavioural responses to objects in the binocular field were examined in frogs with one regenerate and one intact optic nerve. Data were compared to those for normal controls and for frogs with vision via one intact optic nerve. During prey acquisition, frogs with regenerated optic nerves underestimated the distance to the prey on their first strike; as a consequence, the regenerate series made several attempts to achieve a successful prey capture. By contrast, normal frogs and those using only one eye struck accurately at the prey and usually captured it on the first attempt. However, frogs using only one eye struck from a closer distance than either the regenerate or normal series. Frogs with regenerated optic nerves also made more errors than either of the other series when leaping through a set of closely spaced horizontally aligned rods. Our results show that prey capture and the negotiation of horizontally aligned rods is impaired in animals using one regenerated and one intact optic nerve as compared to both normal frogs and those using only one eye. We suggest that the poor visual performance for frogs with one regenerated and one intact optic nerve for tasks presented in the binocular field is related to the integration of a degraded and a normal image within the visual centres.

Optic nerve regenerates but does not restore topographic projections in the lizard Ctenophorus ornatus

In adult fish and amphibians, the severed optic nerve regenerates and visual behaviour is restored. By contrast, optic axons do not regenerate in the more recently evolved birds and mammals. Here we have investigated optic nerve regeneration in a member of the class Reptilia, phylogenetically intermediate between the fish and amphibians and the birds and mammals. We assessed visual recovery anatomically and behaviourally one year after unilateral optic nerve crush in the adult ornate dragon lizard. Ctenophorus ornatus. Ganglion cell densities and numbers of axons in the optic nerve on either side of the crush site indicated that two-thirds of ganglion cells survived axotomy and regrew their axons. However, myelination fell from a mean of 21% in normals to 5.5% and 3%, proximal and distal to the crush, respectively. Anterograde labelling of the entire optic nerve showed that axons regenerated along essentially normal pathways and that the major projection, as in normals, was to the superficial one-third of the contralateral optic tectum. However, localised retinal injections indicated that regenerated projections lacked retinotopic order. Any one retinal region projected to the entire tectum. This feature presumably explains why the experimental lizards consistently appeared blind to stimuli via the regenerated nerve. Our findings indicate that although axons regenerate along essentially normal pathways in adult lizards, conditions within the visual centres do not allow regenerating optic axons to select appropriate central connections. In a wider context, the result suggests that the ability for regenerating central axons to form topographic maps may also have been lost in the more recently evolved vertebrate classes.