Circadian glucocorticoid oscillations preserve a population of adult hippocampal neural stem cells in the aging brain

A decrease in adult hippocampal neurogenesis has been linked to age-related cognitive impairment. However, the mechanisms involved in this age-related reduction remain elusive. Glucocorticoid hormones (GC) are important regulators of neural stem/precursor cells (NSPC) proliferation. GC are released from the adrenal glands in ultradian secretory pulses that generate characteristic circadian oscillations. Here, we investigated the hypothesis that GC oscillations prevent NSPC activation and preserve a quiescent NSPC pool in the aging hippocampus. We found that hippocampal NSPC populations lacking expression of the glucocorticoid receptor (GR) decayed exponentially with age, while GR-positive populations decayed linearly and predominated in the hippocampus from middle age onwards. Importantly, GC oscillations controlled NSPC activation and GR knockdown reactivated NSPC proliferation in aged mice. When modeled in primary hippocampal NSPC cultures, GC oscillations control cell cycle progression and induce specific genome-wide DNA methylation profiles. GC oscillations induced lasting changes in the methylation state of a group of gene promoters associated with cell cycle regulation and the canonical Wnt signaling pathway. Finally, in a mouse model of accelerated aging, we show that disruption of GC oscillations induces lasting changes in dendritic complexity, spine numbers and morphology of newborn granule neurons. Together, these results indicate that GC oscillations preserve a population of GR-expressing NSPC during aging, preventing their activation possibly by epigenetic programming through methylation of specific gene promoters. Our observations suggest a novel mechanism mediated by GC that controls NSPC proliferation and preserves a dormant NSPC pool, possibly contributing to a neuroplasticity reserve in the aging brain.

MiR-30e and miR-181d control radial glia cell proliferation via HtrA1 modulation

The precise mechanisms by which microRNAs (miRNAs) contribute to the dynamic regulation of gene expression during the forebrain development are still partly elusive. Here we show that the depletion of miRNAs in the cerebral cortex and hippocampus, via genetic inactivation of Dicer after the onset of forebrain neurogenesis, profoundly impairs the morphological and proliferative characteristics of neural stem and progenitor cells. The cytoarchitecture and self-renewal potential of radial glial (RG) cells located within the cerebral cortex and the hippocampus were profoundly altered, thus causing a significant derangement of both the post natal dorsal sub-ventricular zone and the dentate gyrus. This effect was attributed to the High-temperature requirement A serine peptidase 1 (HtrA1) gene product whose overexpression in the developing forebrain recapitulated some of the aspects of the Dicer^−/− phenotype. MiR-30e and miR-181d were identified as posttranscriptional negative regulators of HtrA1 by binding to its 3′ untranslated region. In vivo overexpression of miR-30e and miR-181d in Dicer^−/− forebrain rescued RG proliferation defects.