Adult hippocampal neurogenesis is a critical form of cellular plasticity that is greatly influenced by neural activity. Among the neurotransmitters that are widely implicated in regulating this process are serotonin and norepinephrine, levels of which are modulated by stress, depression and clinical antidepressants. However, studies to date have failed to address a direct role for either neurotransmitter in regulating hippocampal precursor activity. Here we show that norepinephrine but not serotonin directly activates self-renewing and multipotent neural precursors, including stem cells, from the hippocampus of adult mice. Mechanistically, we provide evidence that ␤ 3 -adrenergic receptors, which are preferentially expressed on a Hes5-expressing precursor population in the subgranular zone (SGZ), mediate this norepinephrine-dependent activation. Moreover, intrahippocampal injection of a selective ␤ 3 -adrenergic receptor agonist in vivo increases the number of proliferating cells in the SGZ. Similarly, systemic injection of the ␤-adrenergic receptor agonist isoproterenol not only results in enhancement of proliferation in the SGZ but also leads to an increase in the percentage of nestin/glial fibrillary acidic protein double-positive neural precursors in vivo. Finally, using a novel ex vivo "slice-sphere" assay that maintains an intact neurogenic niche, we demonstrate that antidepressants that selectively block the reuptake of norepinephrine, but not serotonin, robustly increase hippocampal precursor activity via ␤-adrenergic receptors. These findings suggest that the activation of neurogenic precursors and stem cells via ␤ 3 -adrenergic receptors could be a potent mechanism to increase neuronal production, providing a putative target for the development of novel antidepressants.

Although a number of growth factors have been shown to be involved in neurogenesis, the role of inflammatory cytokines remains relatively unexplored in the normal brain. Here we investigated the effect of interferon gamma (IFN␥) in the regulation of neural precursor (NP) activity in both the developing and the adult mouse brain. Exogenous IFN␥ inhibited neurosphere formation from the wild-type neonatal and adult subventricular zone (SVZ). More importantly, however, these effects were mirrored in vivo, with mutant mice lacking endogenous IFN␥ displaying enhanced neurogenesis, as demonstrated by an increase in proliferative bromodeoxyuridinelabeled cells in the SVZ and an increased percentage of newborn neurons in the olfactory bulb. Furthermore, NPs isolated from IFN␥ null mice exhibited an increase in self-renewal ability and in the capacity to produce differentiated neurons and oligodendrocytes. These effects resulted from the direct action of IFN␥ on the NPs, as determined by single-cell assays and the fact that nearly all the neurospheres were derived from cells positive for major histocompatibility complex class I antigen, a downstream marker of IFN␥-mediated activation. Moreover, the inhibitory effect was ameliorated in the presence of SVZ-derived microglia, with their removal resulting in almost complete inhibition of NP proliferation. Interestingly, in contrast to the results obtained in the adult, exogenous IFN␥ treatment stimulated neurosphere formation from the embryonic brain, an effect that was mediated by sonic hedgehog. Together these findings provide the first direct evidence that IFN␥ acts as a regulator of the active NP pool in the non-inflammatory brain.

In the search for ways to combat degenerative neurological disorders, neurogenesis-stimulating factors are proving to be a promising area of research. In this study, we show that the hormonal factor prolactin (PRL) can activate a pool of latent precursor cells in the adult mouse hippocampus. Using an in vitro neurosphere assay, we found that the addition of exogenous PRL to primary adult hippocampal cells resulted in an approximate 50% increase in neurosphere number. In addition, direct infusion of PRL into the adult dentate gyrus also resulted in a significant increase in neurosphere number. Together these data indicate that exogenous PRL can increase hippocampal precursor numbers both in vitro and in vivo. Conversely, PRL null mice showed a significant reduction (approximately 80%) in the number of hippocampal-derived neurospheres. Interestingly, no deficit in precursor proliferation was observed in vivo, indicating that in this situation other niche factors can compensate for a loss in PRL. The PRL loss resulted in learning and memory deficits in the PRL null mice, as indicated by significant deficits in the standard behavioral tests requiring input from the hippocampus. This behavioral deficit was rescued by direct infusion of recombinant PRL into the hippocampus, indicating that a lack of PRL in the adult mouse hippocampus can be correlated with impaired learning and memory.

The mineralisation disorder pseudoxanthoma elasticum (PXE) is associated with mutations in the transporter protein ABCC6. Patients with PXE suffer from calcified lesions in the skin, eyes and vasculature, and PXE is related to a more severe vascular calcification syndrome called generalised arterial calcification of infancy (GACI). Mutations in ABCC6 are linked to reduced levels of circulating vitamin K. Here, we describe a mutation in the zebrafish (Danio rerio) orthologue abcc6a, which results in extensive hypermineralisation of the axial skeleton. Administration of vitamin K to embryos was sufficient to restore normal levels of mineralisation. Vitamin K also reduced ectopic mineralisation in a zebrafish model of GACI, and warfarin exacerbated the mineralisation phenotype in both mutant lines. These data suggest that vitamin K could be a beneficial treatment for human patients with PXE or GACI. Additionally, we found that abcc6a is strongly expressed at the site of mineralisation rather than the liver, as it is in mammals, which has significant implications for our understanding of the function of ABCC6.