Regulation of ephrin-A expression in compressed retinocollicular maps

• Authors: T. Tadesse, Georgia State University; Q. Cheng, Georgia State University; M. Xu, Georgia State University; D. J. Baro, Georgia State University; Larry Young, Emory University; S. L. Pallas, Georgia State University
• Journal Title: Developmental Neurobiology
• Volume: Volume 73, Number 4
• Publisher: Wiley: 12 months | 2013-04-01, Pages 274-296
• Type of Work: Article | Post-print: After Peer Review
• Publisher DOI: http://doi.org/10.1002/dneu.22059
• Subjects: Biology, Neuroscience
• Keywords: Science & Technology; Life Sciences & Biomedicine; Developmental Biology; Neurosciences; Neurosciences & Neurology; retinotectal; population matching; topographic map; axon guidance; neural plasticity; GANGLION-CELL AXONS; HAMSTERS MESOCRICETUS-AURATUS; RECEPTOR TYROSINE KINASES; HALF RETINAL PROJECTION; PARTIAL TECTAL ABLATION; SUPERIOR COLLICULUS; RETINOTECTAL PROJECTION; VISUAL-SYSTEM; TOPOGRAPHIC MAPS; GOLDEN-HAMSTER

Abstract

Retinotopic maps can undergo compression and expansion in response to changes in target size, but the mechanism underlying this compensatory process has remained a mystery. The discovery of ephrins as molecular mediators of Sperry's chemoaffinity process allows a mechanistic approach to this important issue. In Syrian hamsters, neonatal, partial (PT) ablation of posterior superior colliculus (SC) leads to compression of the retinotopic map, independent of neural activity. Graded, repulsive EphA receptor/ephrin-A ligand interactions direct the formation of the retinocollicular map, but whether ephrins might also be involved in map compression is unknown. To examine whether map compression might be directed by changes in the ephrin expression pattern, we compared ephrin-A2 and ephrin-A5 mRNA expression between normal SC and PT SC using in situ hybridization and quantitative real-time PCR. We found that ephrin-A ligand expression in the compressed maps was low anteriorly and high posteriorly, as in normal animals. Consistent with our hypothesis, the steepness of the ephrin gradient increased in the lesioned colliculi. Interestingly, overall levels of ephrin-A2 and -A5 expression declined immediately after neonatal target damage, perhaps promoting axon outgrowth. These data establish a correlation between changes in ephrin-A gradients and map compression, and suggest that ephrin-A expression gradients may be regulated by target size. This in turn could lead to compression of the retinocollicular map onto the reduced target. These findings have important implications for mechanisms of recovery from traumatic brain injury.