ABSTRACT
The organization of the visual system is different in birds and mammals. In both, retinal axons project topographically to the visual targets in the brain; but whereas in birds visual fibers from the entire retina decussate at the optic chiasm, in mammals, a number of axons from the temporal retina diverge at the midline to project ipsilaterally. Gain-of-function experiments in chick raised the hypothesis that the transcription factor Foxd1 specifies retinal temporal identity. However, it remains unknown whether Foxd1 is necessary for this function. In mammals, the crucial role of Foxd1 in the patterning of the optic chiasm region has complicated the interpretation of its cell-autonomous function in the retina. Furthermore, target molecules identified for Foxd1 are different in chicks and mice, leading to question the function of Foxd1 in mammals. Here we show that in the mouse, Foxd1 imprints temporal features in the retina such as axonal ipsilaterality and rostral targeting in collicular areas and that EphA6 is a Foxd1 downstream effector that sends temporal axons to the rostral colliculus. In addition, our data support a model in which the desensitization of EphA6 by ephrinA5 in cis is not necessary for the proper functioning of EphA6. Overall, these results indicate that Foxd1 functions as a conserved determinant of temporal identity but reveal that the downstream effectors, and likely their mechanisms of action, are different in mammals and birds.
Subject(s)
Forkhead Transcription Factors/genetics , Forkhead Transcription Factors/physiology , Retina/growth & development , Animals , Axons/physiology , Brain Mapping , Coculture Techniques , DNA/genetics , Electroporation , Ephrin-A5/genetics , Ephrin-A5/physiology , Female , Gene Expression Regulation, Developmental , Geniculate Bodies/cytology , Geniculate Bodies/embryology , Geniculate Bodies/physiology , Immunohistochemistry , In Situ Hybridization , Mice , Mice, Knockout , Plasmids/genetics , Pregnancy , Receptor, EphA6/genetics , Receptor, EphA6/physiology , Retina/embryology , Reverse Transcriptase Polymerase Chain Reaction , Signal Transduction/genetics , Signal Transduction/physiology , Superior Colliculi/cytology , Superior Colliculi/embryology , Superior Colliculi/physiologyABSTRACT
Axons of retinal ganglion cells (RGCs) make a divergent choice at the optic chiasm to cross or avoid the midline in order to project to ipsilateral and contralateral targets, thereby establishing the binocular visual pathway. The zinc-finger transcription factor Zic2 and a member of the Eph family of receptor tyrosine kinases, EphB1, are both essential for proper development of the ipsilateral projection at the mammalian optic chiasm midline. Here, we demonstrate in mouse by functional experiments in vivo that Zic2 is not only required but is also sufficient to change the trajectory of RGC axons from crossed to uncrossed. In addition, our results reveal that this transcription factor regulates the expression of EphB1 in RGCs and also suggest the existence of an additional EphB1-independent pathway controlled by Zic2 that contributes to retinal axon divergence at the midline.
Subject(s)
Axons/physiology , Nuclear Proteins/physiology , Optic Chiasm/cytology , Receptor, EphB1/physiology , Transcription Factors/physiology , Animals , Female , Green Fluorescent Proteins/metabolism , Humans , Mice , Nuclear Proteins/biosynthesis , Nuclear Proteins/genetics , Optic Chiasm/embryology , Retinal Ganglion Cells/cytology , Retinal Ganglion Cells/metabolism , Transcription Factors/biosynthesis , Transcription Factors/geneticsABSTRACT
BACKGROUND: The neural retina is a highly structured tissue of the central nervous system that is formed by seven different cell types that are arranged in layers. Despite much effort, the genetic mechanisms that underlie retinal development are still poorly understood. In recent years, large-scale genomic analyses have identified candidate genes that may play a role in retinal neurogenesis, axon guidance and other key processes during the development of the visual system. Thus, new and rapid techniques are now required to carry out high-throughput analyses of all these candidate genes in mammals. Gene delivery techniques have been described to express exogenous proteins in the retina of newborn mice but these approaches do not efficiently introduce genes into the only retinal cell type that transmits visual information to the brain, the retinal ganglion cells (RGCs). RESULTS: Here we show that RGCs can be targeted for gene expression by in utero electroporation of the eye of mouse embryos. Accordingly, using this technique we have monitored the morphology of electroporated RGCs expressing reporter genes at different developmental stages, as well as their projection to higher visual targets. CONCLUSION: Our method to deliver ectopic genes into mouse embryonic retinas enables us to follow the course of the entire retinofugal pathway by visualizing RGC bodies and axons. Thus, this technique will permit to perform functional studies in vivo focusing on neurogenesis, axon guidance, axon projection patterning or neural connectivity in mammals.
