Members of the cyclin-dependent kinase (CDK)-inhibitory protein (CIP)/kinase-inhibitory protein (KIP) family of cyclin-dependent kinase inhibitors regulate proliferation and cell cycle exit of mammalian cells. In the adult brain, the CIP/KIP protein p27 kip1 has been related to the regulation of intermediate progenitor cells located in neurogenic niches. Here, we uncover a novel function of p27 kip1 in the adult hippocampus as a dual regulator of stem cell quiescence and of cell-cycle exit of immature neurons. In vivo, p27 kip1 is detected in radial stem cells expressing SOX2 and in newborn neurons of the dentate gyrus. In vitro, the Cdkn1b gene encoding p27 kip1 is transcriptionally upregulated by quiescence signals such as BMP4. The nuclear accumulation of p27 kip1 protein in adult hippocampal stem cells encompasses the BMP4-induced quiescent state and its overexpression is able to block proliferation. p27 kip1 is also expressed in immature neurons upon differentiation of adult hippocampal stem cell cultures. Loss of p27 kip1 leads to an increase in proliferation and neurogenesis in the adult dentate gyrus, which results from both a decrease in the percentage of radial stem cells that are quiescent and a delay in cell cycle exit of immature neurons. Analysis of animals carrying a disruption in the cyclin-CDK interaction domain of p27 kip1 indicates that the CDK inhibitory function of the protein is necessary to control the activity of radial stem cells. Thus, we report that p27 kip1 acts as a central player of the molecular program that keeps adult hippocampal stem cells out of the cell cycle.

Summary The existence of axons extending from one retina to the other has been reported during perinatal development in different vertebrates. However, it has been thought that these axons are either a labeling artifact or misprojections. Here, we show unequivocally that a small subset of retinal ganglion cells (RGCs) project to the opposite retina and that the guidance receptor Unc5c, expressed in the retinal region where the retinal-retinal (R-R) RGCs are located, is necessary and sufficient to guide axons to the opposite retina. In addition, Netrin1, an Unc5c ligand, is expressed in the ventral diencephalon in a pattern that is consistent with impeding the growth of Unc5c-positive retinal axons into the brain. We also have generated a mathematical model to explore the formation of retinotopic maps in the presence and absence of a functional connection between both eyes. This model predicts that an R-R connection is required for the bilateral coordination of axonal refinement in species where refinement depends upon spontaneous retinal waves. Consistent with this idea, the retinal expression of Unc5c correlates with the existence and size of an R-R projection in different species and with the extent of axonal refinement in visual targets. These findings demonstrate that active guidance drives the formation of the R-R projection and suggest an important role for these projections in visual mapping to ensure congruent bilateral refinement.

The formation of the visual system is a complex multistep process that includes proper assembly between retinal ganglion cell (RGC) axon terminals and their relay neurons in the different visual nuclei of the brain. RGC axons reach the main image-forming nuclei (IFN) —the superior colliculus and the lateral geniculate nucleus— at perinatal stages and extensively arborize to then refine throughout the first postnatal weeks. Spontaneous activity generated in the immature retina plays an essential role in the fine tune refinement of exhuberant axonal arborizations but the molecular mechanisms underlying this activity-dependent remodeling process remain poorly characterized. RGC axons, in addition to innervate IFN, target non-image forming nuclei (NIFN), but the impact of spontaneous retinal activity in the development of these accessory nuclei has not yet been described. Here, by genetically altering spontaneous activity in the RGCs of mice we demonstrate that correlated retinal activity also shapes the connectivity of the non-image forming circuit and identify the transcriptional programs modulating this process. Together, our data contribute to a better understanding of the molecular mechanisms ruling activity-dependent axon refining in the building of visual circuits.

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