The initial stages of development of the retinocollicular projection in the wallaby (Macropus eugenii): distribution of ganglion cells in the retina and their axons in the superior colliculus

Overexpression of Ten-m3 in the retina alters ipsilateral retinocollicular projections in the wallaby (Macropus eugenii)

Retinal projections to the superior colliculus are organised into retinotopic maps. Binocular vision requires that inputs from the two eyes map in register with each other. Studies in mice lacking Ten-m3, a homophilic transmembrane protein, indicate that it plays a key role in this process by influencing ipsilateral projections. The postnatal, ex utero development of the wallaby allows the targeted manipulation of molecules of interest during development. The distribution of mRNA for Ten-m3 in the retina and superior colliculus of the wallaby, and the effects of its spatiotemporally restricted retinal overexpression was investigated, in particular on the mapping of ipsilateral projections. Quantitative polymerase chain reaction found that Ten-m3 mRNA is expressed at relatively higher levels in the retina and colliculus early in development. Further, it is higher in ventral than dorsal retina, and increased in the retinotopically corresponding medial compared to lateral superior colliculus. In situ hybridisation demonstrated an increasing dorsoventral gradient in retinal ganglion cells was matched to an increasing lateromedial gradient in the superior colliculus. Overexpression of Ten-m3 by in vivo retinal electroporation produced an increase in ipsilateral projections to the binocular rostromedial colliculus, fitting with the proposal that Ten-m3 mediates mapping by attractant homophilic interactions. Retrograde labelling of the projection from this region suggested that overexpression produces a shift in the axons of existing ipsilaterally projecting ganglion cells rather than a rerouting of the axons of contralaterally projecting cells. Retinal manipulation of Ten-m3 levels produces changes in ipsilateral mapping, supporting a role for it in binocular mapping.

Retinocollicular synaptogenesis and synaptic transmission during formation of the visual map in the superior colliculus of the wallaby (Macropus eugenii)

Spontaneous retinal activity has been implicated in the development of the topographic map in the superior colliculus (SC) but a direct demonstration that it reaches the colliculus is lacking. Here we investigate when the retinocollicular projection is capable of transmitting information from the retina in a marsupial mammal, the wallaby (Macropus eugenii). The projection develops postnatally, allowing in vivo analysis throughout development. Quantification of retinocollicular synaptogenesis has been combined with electrophysiology of the development and characteristics of retinocollicular transmission, including in vivo and in vitro recording in the same animals. Prior to postnatal day (P) 12-14 in vitro recording detected only presynaptic activity in retinal axons in the colliculus, in response to stimulation of the optic nerve. Postsynaptic responses, comprising both N-methyl-d-aspartate (NMDA) and non-NMDA responses, were first detected in vitro at P12-14 and retinal synapses were identified. In contrast, postsynaptic responses to optic nerve stimulation could not be detected in vivo until P39, around the time that retinal axons begin arborizing. Around this age density and numbers of total synapses began increasing in the retinorecipient layers of the colliculus. By P55-64, the numbers of retinal synapses had increased significantly and density and numbers of retinal and total synapses continued to increase up to P94-99. During this time the map is undergoing refinement and degenerating axons and synapses were present. The discrepancy between in vitro and in vivo onset of functional connections raises the question of when retinal activity reaches collicular cells in the intact, unanaesthetized animal and this will require investigation.

Analysis of EphB receptors and their ligands in the developing retinocollicular system of the wallaby reveals dynamic patterns of expression in the retina

The expression of EphB1 and B2 receptors and ephrins-B1, -B2 and -B3 in the retina and superior colliculus of the wallaby (Macropus eugenii) was examined during the development of the retinocollicular projection, using reverse transcription-polymerase chain reaction and immunohistochemistry. There was an early transient differential expression of EphB2 that was higher in ventral retina and restricted to the outer neuroblast layer, whereas a high ventral to low dorsal gradient of ephrin-B2 expression occurred there throughout the study period. However, there was no dorsoventral gradient of receptors or ligands in retinal ganglion cells or a mediolateral gradient of ephrins in the colliculus. These findings suggest a limited role for these molecules in topographic mapping across the mediolateral colliculus in the wallaby. Early in retinal development there is a complementary pattern of expression of ephrin-B1 and -B2 in the outer neuroblast layer that overlaps with expression of EphB2. Ganglion and amacrine cells also express EphB2. As development proceeds subpopulations of putative horizontal and bipolar cells, also expressing EphB2, come to reside in the inner nuclear layer and ephrin-B1 is expressed throughout the outer nuclear layer. At the same time cells expressing ephrin-B2, and subpopulations of horizontal and bipolar cells come to reside in the inner nuclear layer and there is a corresponding decrease in ephrin-B2 expression in the outer nuclear layer. This pattern of coexpression of receptors and ligands suggests a role for them in cell migration and maintenance of laminar boundaries.

Topographic-specific axon branching controlled by ephrin-As is the critical event in retinotectal map development

The retinotectal projection is the predominant model for studying molecular mechanisms controlling development of topographic axonal connections. Our analyses of topographic mapping of retinal ganglion cell (RGC) axons in chick optic tectum indicate that a primary role for guidance molecules is to regulate topographic branching along RGC axons, a process that imposes unique requirements on the molecular control of map development. We show that topographically appropriate connections are established exclusively by branches that form along the axon shaft. Initially, RGC axons overshoot their appropriate termination zone (TZ) along the anterior-posterior (A-P) tectal axis; temporal axons overshoot the greatest distance and nasal axons the least, which correlates with the nonlinear increasing A-P gradient of ephrin-A repellents. In contrast, branches form along the shaft of RGC axons with substantial A-P topographic specificity. Topography is enhanced through the preferential arborization of appropriately positioned branches and elimination of ectopic branches. Using a membrane stripe assay and time-lapse microscopy, we show that branches form de novo along retinal axons. Temporal axons preferentially branch on their topographically appropriate anterior tectal membranes. After the addition of soluble EphA3-Fc, which blocks ephrin-A function, temporal axons branch equally on anterior and posterior tectal membranes, indicating that the level of ephrin-As in posterior tectum is sufficient to inhibit temporal axon branching and generate branching specificity in vitro. Our findings indicate that topographic branch formation and arborization along RGC axons are critical events in retinotectal mapping. Ephrin-As inhibit branching along RGC axons posterior to their correct TZ, but alone cannot account for topographic branching and must cooperate with other molecular activities to generate appropriate mapping along the A-P tectal axis.

Graded expression of EphA3 in the retina and ephrin-A2 in the superior colliculus during initial development of coarse topography in the wallaby retinocollicular projection

We describe the expression of EphA3 and EphA7 receptors and ephrin-A2 ligand in the retina and the superior colliculus during the development of the retinocollicular projection in the marsupial wallaby (Macropus eugenii), using immunoblotting and immunohistochemistry. EphA3 in the retina was in a striking, low central to high peripheral gradient, superimposed on which was a high temporal to low nasal level of expression. This distribution was evident from postnatal day 30, when axons are growing into the colliculus and forming a coarsely organized topographic projection, to postnatal day 65, when axons have arborized in their correct retinotopic positions. EphA7 showed a shallow centroperipheral gradient with no nasotemporal differences in expression. In the superior colliculus no rostrocaudal differences in distribution were detected for either of these receptors. Ephrin-A2 was distributed in a gradient increasing from the rostral to the caudal pole in the superficial layers of the superior colliculus only up to postnatal day 30. Ephrin-A2 was evenly distributed in the retina throughout development of the projection. Expression of EphA3 in the retina increased, while the expression of ephrin-A2 in the colliculus was downregulated over time. The graded expression of EphA3 and ephrin-A2 early in the development of the projection suggests that they play a role in establishment of coarse topography of retinal axons along the rostrocaudal axis of the superior colliculus. However, the gradients were not complementary, meaning that EphA3 alone cannot mediate the repulsive interactions with ephrin-A2 that have been postulated to underlie formation of the topographic map.

Marsupial retinocollicular system shows differential expression of messenger RNA encoding EphA receptors and their ligands during development

The protracted development of the wallaby (Macropus eugenii) has allowed study of messenger RNAs encoding Eph receptors EphA3 and EphA7 and ligands ephrin-A2 and -A5 in the retina and superior colliculus at intervals throughout the development of the retinocollicular projection: from birth, before retinal innervation, to postnatal day 95, when the projection is mature. Reverse transcription-polymerase chain reaction showed messenger RNAs for both receptors and ligands were expressed at all ages. EphA7 was expressed more highly in the rostral superior colliculus. Ephrin-A2 and -A5 were expressed more highly in the caudal colliculus. EphA3 was expressed in a complementary manner, more highly in temporal than in nasal retina. There are higher levels of expression of the ligands when the projection is only coarsely topographically organised. This suggests a role for them and their receptor EphA3 in this stage, by repulsive interactions which restrict temporal axons to rostral superior colliculus. This is the first account in a marsupial mammal of the appearance of this molecular family, substantiating its ubiquitous role in topographically organised neuronal connections. Nevertheless, expression is not the same as in the mouse, suggesting differences in the details of topographic coding between species.

Conduction and synaptic transmission in the optic nerve and the superior colliculus during development of the retinocollicular projection in the wallaby (Macropus eugenii)

When do the developing connections between mammalian retinal ganglion cells and the superior colliculus become functional? Evoked potentials elicited by optic nerve stimulation in the pouch young of the wallaby were used to answer the question. Up to 42 days after birth, the evoked potentials in the colliculus appeared to be generated by axon conduction. Synaptic activity was first recorded from the rostral colliculus at 45 days, and was found to be progressively more caudal, spreading to cover the colliculus, by 65 days. From the earliest indication of synaptic activity until eye opening at 140 days, current source density (CSD) analysis consistently showed the same basic pattern: an initial deep sink from synaptic activity of fast (Y type) fibres, and a more superficial longer-latency sink from slower (W type) fibres. All features became more clearly delineated with age. The indirect retinocorticocollicular connection appeared between 134 days and 146 days. The ability of optic nerve fibres to sustain action potentials precedes their formation of functional synapses with collicular neurons, which happens abruptly at three months before eye opening. CSD analysis showed that the relationship between the conduction velocity of optic nerve fibres and their depth of termination is evident from the first signs of synapse formation.