Eph/ephrin gradients in the retinotectal system of Rana pipiens: developmental and adult expression patterns

DSCAM is differentially patterned along the optic axon pathway in the developing Xenopus visual system and guides axon termination at the target

BACKGROUND

The Xenopus retinotectal circuit is organized topographically, where the dorsal-ventral axis of the retina maps respectively on to the ventral-dorsal axis of the tectum; axons from the nasal-temporal axis of the retina project respectively to the caudal-rostral axis of the tectum. Studies throughout the last two decades have shown that mechanisms involving molecular recognition of proper termination domains are at work guiding topographic organization. Such studies have shown that graded distribution of molecular cues is important for topographic mapping. However, the complement of molecular cues organizing topography along the developing optic nerve, and as retinal axons cross the chiasm and navigate towards and innervate their target in the tectum, remains unknown. Down syndrome cell adhesion molecule (DSCAM) has been characterized as a key molecule in axon guidance, making it a strong candidate involved in the topographic organization of retinal fibers along the optic path and at their target.

METHODS

Using a combination of whole-brain clearing and immunohistochemistry staining techniques we characterized DSCAM expression and the projection of ventral and dorsal retinal fibers starting from the eye, following to the optic nerve and chiasm, and into the terminal target in the optic tectum in Xenopus laevis tadpoles. We then assessed the effects of DSCAM on the establishment of retinotopic maps through spatially and temporally targeted DSCAM knockdown on retinal ganglion cells (RGCs) with axons innervating the optic tectum.

RESULTS

Highest expression of DSCAM was localized to the ventral posterior region of the optic nerve and chiasm; this expression pattern coincides with ventral fibers derived from ventral RGCs. Targeted downregulation of DSCAM expression on ventral RGCs affected the segregation of medial axon fibers from their dorsal counterparts within the tectal neuropil, indicating that DSCAM plays a role in retinotopic organization.

CONCLUSION

These findings together with previous studies demonstrating cell-autonomous roles for DSCAM during the development of pre- and postsynaptic arbors in the Xenopus retinotectal circuit indicates that DSCAM exerts multiple roles in coordinating axon targeting and structural connectivity in the developing vertebrate visual system.

Eph Receptors and Ephrins in Retinal Diseases

Retinal diseases are the leading cause of irreversible blindness. They affect people of all ages, from newborns in retinopathy of prematurity, through age-independent diabetic retinopathy and complications of retinal detachment, to age-related macular degeneration (AMD), which occurs mainly in the elderly. Generally speaking, the causes of all problems are disturbances in blood supply, hypoxia, the formation of abnormal blood vessels, and fibrosis. Although the detailed mechanisms underlying them are varied, the common point is the involvement of Eph receptors and ephrins in their pathogenesis. In our study, we briefly discussed the pathophysiology of the most common retinal diseases (diabetic retinopathy, retinopathy of prematurity, proliferative vitreoretinopathy, and choroidal neovascularization) and collected available research results on the role of Eph and ephrins. We also discussed the safety aspect of the use of drugs acting on Eph and ephrin for ophthalmic indications.

Molecular guidance of retinotopic map development in the midbrain

Topographic maps are utilized in many sensory and motor systems to efficiently transfer information between brain regions. The retina's projection to the superior colliculus has served as a model for the identification of molecular cues and mechanistic strategies by which topographic maps are formed. Evidence from both in vitro and in vivo studies points to graded cell surface cues playing a central role, but support for axon-axon competition and selective degeneration have also been advanced recently. In combination with mathematical models, these studies suggest that topographic maps are established using a complex combination of strategies to ensure precise connectivity.

Expression patterns of Ephs and ephrins throughout retinotectal development in Xenopus laevis

The Eph family of receptor tyrosine kinases and their ligands the ephrins play an essential role in the targeting of retinal ganglion cell axons to topographically correct locations in the optic tectum during visual system development. The African claw-toed frog Xenopus laevis is a popular animal model for the study of retinotectal development because of its amenability to live imaging and electrophysiology. Its visual system undergoes protracted growth continuing beyond metamorphosis, yet little is known about ephrin and Eph expression patterns beyond stage 39 when retinal axons first arrive in the tectum. We used alkaline phosphatase fusion proteins of EphA3, ephrin-A5, EphB2, and ephrin-B1 as affinity probes to reveal the expression patterns of ephrin-As, EphAs, ephrin-Bs, and EphBs, respectively. Analysis of brains from stage 40 to adult frog revealed that ephrins and Eph receptors are expressed throughout development. As observed in other species, staining for ephrin-As displayed a high caudal to low rostral expression pattern across the tectum, roughly complementary to the expression of EphAs. In contrast with the prevailing model, EphBs were found to be expressed in the tectum in a high dorsal to low ventral gradient in young animals. In animals with induced binocular tectal innervation, ocular dominance bands of alternating input from the two eyes formed in the tectum; however, ephrin-A and EphA expression patterns were unmodulated and similar to those in normal frogs, confirming that the segregation of axons into eye-specific stripes is not the consequence of a respecification of molecular guidance cues in the tectum.

Eph and ephrin signaling in the formation of topographic maps

The axonal connections between the retina and its midbrain target, the superior colliculus (SC), is mapped topographically, such that the spatial relationships of cell bodies in the retina are maintained when terminating in the SC. Topographic map development uses a Cartesian mapping system such that each axis of the retina is mapped independently. Along the nasal-temporal mapping axis, EphAs and ephrin-As, are graded molecular cues required for topographic mapping while the dorsal-ventral axis is mapped in part via EphB and ephrin-Bs. Because both Ephs and ephrins are cell surface molecules they can signal in the forward and reverse directions. Eph/ephrin signaling leads to changes in cytoskeletal dynamics that lead to actin depolymerization and endocytosis guiding axons via attraction and repulsion.

Region-specific contribution of ephrin-B and Wnt signaling to receptive field plasticity in developing optic tectum

Ephrin-B/EphB and Wnts are known to regulate synapse maturation and plasticity, besides serving as axon guidance molecules, but the relevance of such synaptic regulation to neural circuit functions in vivo remains unclear. In this study, we have examined the role of ephrin-B and Wnt signaling in regulating visual experience-dependent and developmental plasticity of receptive fields (RFs) of tectal cells in the developing Xenopus optic tectum. We found that repetitive exposure to unidirectional moving visual stimuli caused varying degrees of shift in the RFs in different regions of the tectum. By acute perfusion of exogenous antagonists and inducible transgene expression, we showed that ephrin-B signaling in presynaptic retinal ganglion cells and Wnt secretion from tectal cells are specifically responsible for the enhanced visual stimulation-induced changes in neuronal responses and RFs in the ventral and dorsal tectum, respectively. Thus, ephrin-B and Wnt signaling contribute to region-specific plasticity of visual circuit functions.