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Tyler Schwend, Peter Y. Lwigale, Gary W. Conrad; Robo-Slit Signaling Regulates Avian Cornea Innervation. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1975.
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© ARVO (1962-2015); The Authors (2016-present)
The cornea is the most densely innervated tissue on the surface of the body. Its nerves serve a sensory role and deliver critical neurotrophins that promote tissue maintenance and repair. Cornea innervation during avian embryonic development is highly coordinated, wherein axons extend from the trigeminal ganglion (TG) and reach the cornea periphery by embryonic day (E)5, but are repelled from entering for many days, instead forming a nerve ring around the cornea margin prior to entering the corneal stroma on E9. Initial nerve repulsion from the cornea is at least partly mediated by neurorepellant guidance molecules secreted by the lens, which lies adjacent to the cornea during development and releases factors that diffuse into the corneal stroma, causing it to be non-permissive to axons. Slit ligands and their Robo receptors have been implicated in axonal repulsion and its family members are expressed in ocular tissues and trigeminal axon cell bodies during development, making them an ideal candidate for initial nerve repulsion to the cornea.
Developing chick corneas, lens and TGs were isolated from E5 - E8 of incubation. Gene expression for Slit ligands 1-3 and their receptors Robo 1-2 were assessed in ocular and TG tissues respectively by RT-PCR and in situ hybridization. To determine the nature of Robo-Slit regulation on trigeminal sensory axons, TG explants (as a source of neurons) and lens were co-cultured in collagen gels in the presence and absence of inhibitors of Robo/Slit signaling. Trigeminal axons in organ cultures were visualized by immunostaining with neuron specific beta III tubulin antibody.
Gene expression analysis reveals that Slit ligands 1-3 and Robo 1-2 receptors are concurrently expressed in ocular tissues (cornea and lens) and TG tissue, respectively, during early development. Early functional study results using organ co-cultures suggest that Robo1, but not Robo2, is involved in mediating axon repulsion by the lens during cornea innervation. Co-culture experiments are currently underway using function-blocking antibodies to specific Slit ligands in order to determine the potential Slit(s) proteins produced by the lens that interact with Robo1 to promote trigeminal axon repulsion.
These findings suggest that Robo-Slit signaling interactions occur between the lens and trigeminal axons during cornea innervation and the pathway is involved in the initial repulsion of trigeminal axons from the cornea.
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