MicroRNA-124a (miR-124a) is the most abundant microRNA expressed in the vertebrate CNS. Despite past investigations into the role of miR-124a, inconsistent results have left the in vivo function of miR-124a unclear. We examined the in vivo function of miR-124a by targeted disruption of Rncr3 (retinal non-coding RNA 3), the dominant source of miR-124a. Rncr3(-/-) mice exhibited abnormalities in the CNS, including small brain size, axonal mis-sprouting of dentate gyrus granule cells and retinal cone cell death. We found that Lhx2 is an in vivo target mRNA of miR-124a. We also observed that LHX2 downregulation by miR-124a is required for the prevention of apoptosis in the developing retina and proper axonal development of hippocampal neurons. These results suggest that miR-124a is essential for the maturation and survival of dentate gyrus neurons and retinal cones, as it represses Lhx2 translation.

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Dystroglycan (DG) is a key component of the dystrophin-glycoprotein complex (DGC)at the neuromuscular junction postsynapse. In the mouse retina, the DGC is localized at the presynapse of photoreceptor cells, however, the function of presynaptic DGC is poorly understood. Here, we developed and analyzed retinal photoreceptor-specific DG conditional knock-out (DG CKO) mice. We found that the DG CKO retina showed a reduced amplitude and a prolonged implicit time of the ERG b-wave. Electron microscopic analysis revealed that bipolar dendrite invagination into the photoreceptor terminus is perturbed in the DG CKO retina. In the DG CKO retina, pikachurin, a DG ligand in the retina, is markedly decreased at photoreceptor synapses. Interestingly, in the Pikachurin Ϫ/Ϫ retina, the DG signal at the ribbon synaptic terminus was severely reduced, suggesting that pikachurin is required for the presynaptic accumulation of DG at the photoreceptor synaptic terminus, and conversely DG is required for pikachurin accumulation. Furthermore, we found that overexpression of pikachurin induces formation and clustering of a DG-pikachurin complex on the cell surface. The Laminin G repeats of pikachurin, which are critical for its oligomerization and interaction with DG, were essential for the clustering of the DG-pikachurin complex as well. These results suggest that oligomerization of pikachurin and its interaction with DG causes DG assembly on the synapse surface of the photoreceptor synaptic terminals. Our results reveal that the presynaptic interaction of pikachurin with DG at photoreceptor terminals is essential for both the formation of proper photoreceptor ribbon synaptic structures and normal retinal electrophysiology.

A number of homeodomain transcription factors, which play significant roles in retinal development, have been identified in vertebrates (1-4). Rax is a homeodomain transcription factor that is essential for various processes in vertebrate retinal development (5). The Rax gene was first identified as a paired-type homeobox gene expressed in the optic vesicle and the presumptive diencephalon area in the early mouse embryo (6, 7). Rax is evolutionarily well conserved from Drosophila melanogaster to humans. Rax is highly expressed in retinal progenitor cells (RPCs), and its expression in the retina gradually decreases as RPCs become postmitotic and begin to differentiate. Rax-null mutant mice exhibit a reduction of brain size and an absence of the optic vesicle (5, 7). Mutations in human RAX are associated with anophthalmia and microphthalmia (8, 9). Rax overexpression promotes the proliferation of RPCs in frogs and zebra fish (7,(10)(11)(12)(13). In addition to the function in RPCs, Rax plays significant roles in the development of photoreceptor cells and Müller glial cells (14)(15)(16)(17)(18)(19).Rax paralog genes have been identified in various vertebrate species (20)(21)(22). In Xenopus laevis, two Rax genes (xRx and xRx-L/xRx2) have been identified (7, 21), and in zebra fish, three Rax genes (zRx1 to zRx3) have been isolated (7). Interestingly, the expression pattern of zebra fish Rx3 showed more similarity to that of frog and mouse Rax genes than to that of the zebra fish Rx1 and Rx2 genes (23). In chicks, two Rax genes (cRax and cRaxL/ cRax2) have been identified (20). The chick Rax2 gene is expressed in both retinal progenitor cells and early-developing photoreceptors, while chick Rax is predominantly expressed in retinal progenitor cells. It was also reported that chick Rax2 is implicated in cone photoreceptor differentiation and that the expression of a putative dominant negative allele of a chick Rax2 gene caused a significant reduction in the level of expression of cone photoreceptor genes (20). Human RAX2/QRX, which is expressed in the outer nuclear layer (ONL) and inner nuclear layer (INL) of the adult human retina, was identified to be a PCE-1-binding protein by acting synergistically with CRX and NRL to modulate the expression of photoreceptor genes. Monkey, cow, and dog genomes also contain two Rax genes. On the other hand, the Rax2 gene is absent from mouse and rat genomes (22). This raises the question of whether mouse Rax plays an essential role in photoreceptor development during postnatal stages like human Rax2 does.In the current study, we investigated a functional role for Rax in postnatal mouse retinas, which contain a single Rax gene. We report that mouse Rax modulates the expression of photoreceptor genes in the postnatal retina by interacting with Crx. Conditional ablation of Rax in postnatal photoreceptors led to a significant decrease in the level of expression of rod and cone genes and to cone photoreceptor cell death, suggesting that Rax is essential for the maturation of rods and co...