Proceedings of The Physiological Society
King's College London (2011) Proc Physiol Soc 22, PC35
Retinal ganglion cell axons need microRNA function for correct pathfinding during mouse visual system development
N. A. Maiorano1
1. MRC Centre for Developmental Neurobiology, Kings College London, London, United Kingdom.
The visual system has been largely investigated to identify the molecular mechanisms controlling the formation of neuronal connectivity during development. In animals with binocular vision, the optic chiasm is a major decision point where retinal ganglion cell (RGC) axons arising from the eyes sort into ipsi- and contralateral projections. In mice, about 97% of the axons cross to the contralateral side, whereas the rest stays ipsilaterally, projecting towards the lateral geniculate nucleus and then into the superior colliculus. Here, RGC axons form connections that obey the rules of a defined topographic map. In the last years intensive studies in the field highlighted the roles of several polypeptide-encoding genes controlling the different morphogenetic subroutines, which ensure the correct wiring of the visual system. Recently, through the use of conditional inactivation of RNase III enzyme Dicer, we and others demonstrated that microRNAs (miRNAs), a class of small non-coding RNAs, are fundamental regulators of retinal histogenesis and axon guidance decisions at the optic chiasm. Our results show that mouse embryos with an early loss of Dicer -from E7.5- in the retina and in the ventral diencephalon, exhibit a microphthalmia phenotype associated to a high rate of apoptosis during neurogenesis. In these mutant embryos we also noticed a significant increase of ipsilateral projections and defasciculated axons at the optic chiasm and in the retina. Moreover, a considerable number of RGC axons aberrantly project from one eye into the other or enter the diencephalon ectopically. Spatiotemporal transcripts regulation of genes involved in patterning and axon guidance due to the direct or indirect action of miRNAs, both in the optic chiasm and in the retina, may lead to the detected phenotypes in mice conditionally Dicer deleted. Nevertheless, using in situ hybridization and immunohistochemistry we find that Dicer mutant retinae and optic chiasm are patterned normally along both axes without showing any changes of guidance molecule expression pattern. Similarly, the number and location of Zic2-expressing RGCs is unchanged in Dicer mutant mice, compared to wildtype, suggesting that the specification of the ipsilateral retinal domain is not controlled by miRNAs. At the moment, in order to define a possible cell-autonomous way-of-action of miRNAs during axon outgrowth, we are analyzing mouse embryos with Dicer deletion only in subset cells of retina from E10.5. This will help to clarify the molecular pathways involved in RGC axon extension, pathfinding and synapse formation. Our work presents a role for miRNAs as a linchpin for the establishment of the visual circuitry during development. Support: MRC G0601182 and Wellcome Trust 087883/08
Where applicable, experiments conform with Society ethical requirements