title: cAMP-induced expression of neuropilin1 promotes retinal axon crossing in the zebrafish optic chiasm.
abstract: Growing axons navigate a complex environment as they respond to attractive and repellent guidance cues. Axons can modulate their responses to cues through a G-protein-coupled, cAMP-dependent signaling pathway. To examine the role of G-protein signaling in axon guidance in vivo, we used the GAL4/UAS system to drive expression of dominant-negative heterotrimeric G-proteins (DNG) in retinal ganglion cells (RGCs) of embryonic zebrafish. Retinal axons normally cross at the ventral midline and project to the contralateral tectum. Expression of DNGα(S) in RGCs causes retinal axons to misproject to the ipsilateral tectum. These errors resemble misprojections in adcy1, adcy8, nrp1a, sema3D, or sema3E morphant embryos, as well as in sema3D mutant embryos. nrp1a is expressed in RGCs as their axons extend toward and across the midline. sema3D and sema3E are expressed adjacent to the chiasm, suggesting that they facilitate retinal midline crossing. We demonstrate synergistic induction of ipsilateral misprojections between adcy8 knockdown and transgenic DNGα(S) expression, adcy8 and nrp1a morphants, or nrp1a morphants and transgenic DNGα(S) expression. Using qPCR analysis, we show that either transgenic DNGα(S)-expressing embryos or adcy8 morphant embryos have decreased levels of nrp1a and nrp1b mRNA. Ipsilateral misprojections in adcy8 morphants are corrected by the expression of an nrp1a rescue construct expressed in RGCs. These findings are consistent with the idea that elevated cAMP levels promote Neuropilin1a expression in RGCs, increasing the sensitivity of retinal axons to Sema3D, Sema3E, or other neuropilin ligands at the midline, and consequently facilitate retinal axon crossing in the chiasm.
authors.affiliation: Department of Neuroscience, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
authors.forename: Alison L
authors.initials: AL
authors.lastname: Dell
authors.forename: Emma
authors.initials: E
authors.lastname: Fried-Cassorla
authors.forename: Hong
authors.initials: H
authors.lastname: Xu
authors.forename: Jonathan A
authors.initials: JA
authors.lastname: Raper
mesh_terms: D000818:Animals; D030801:Animals, Genetically Modified; D001369:Axons / Q000378:metabolism*; D000242:Cyclic AMP / Q000096:biosynthesis* / Q000235:genetics; D005260:Female; D018507:Gene Expression Regulation, Developmental; D008297:Male; D039942:Neuropilin-1 / Q000096:biosynthesis* / Q000235:genetics; D009897:Optic Chiasm / Q000196:embryology / Q000378:metabolism*; D012160:Retina / Q000196:embryology / Q000378:metabolism*; D012165:Retinal Ganglion Cells / Q000378:metabolism; D014795:Visual Pathways / Q000196:embryology / Q000378:metabolism; D015027:Zebrafish
chemical_list: D039942:Neuropilin-1; D000242:Cyclic AMP