Date Published: April , 2012
Publisher: Blackwell Publishing Ltd
Author(s): Eva Terzibasi Tozzini, Mario Baumgart, Giorgia Battistoni, Alessandro Cellerino.
We studied adult neurogenesis in the short-lived annual fish Nothobranchius furzeri and quantified the effects of aging on the mitotic activity of the neuronal progenitors and the expression of glial fibrillary acid protein (GFAP) in the radial glia. The distribution of neurogenic niches is substantially similar to that of zebrafish and adult stem cells generate neurons, which persist in the adult brain. As opposed to zebrafish, however, the N. furzeri genome contains a doublecortin (DCX) gene. Doublecortin is transiently expressed by newly generated neurons in the telencephalon and optic tectum (OT). We also analyzed the expression of the microRNA miR-9 and miR-124 and found that they have complementary expression domains: miR-9 is expressed in the neurogenic niches of the telencephalon and the radial glia of the OT, while miR-124 is expressed in differentiated neurons. The main finding of this paper is the demonstration of an age-dependent decay in adult neurogenesis. Using unbiased stereological estimates of cell numbers, we detected an almost fivefold decrease in the number of mitotically active cells in the OT between young and old age. This reduced mitotic activity is paralleled by a reduction in DCX labeling. Finally, we detected a dramatic up-regulation of GFAP in the radial glia of the aged brain. This up-regulation is not paralleled by a similar up-regulation of S100B and Musashi-1, two other markers of the radial glia. In summary, the brain of N. furzeri replicates two typical hallmarks of mammalian aging: gliosis and reduced adult neurogenesis.
The continued production of neurons throughout adulthood, in at least in some circumscribed brain regions, has been observed in all vertebrate species so far studied. In mammals, adult neurogenesis is restricted to two principal stem cell niches in the telencephalon: the subependymal zone (or subventricular zone, SVZ) which generates GABAergic and dopaminergic interneurons of the olfactory bulb and the subgranular zone (SGZ) of the dentate gyrus. These areas contain adult neuronal stem cells (aNSCs) of glial phenotype, which are able to self-renew. aNSCs generate an intermediate precursor, which gives rise to a transient amplifying neuroblast and finally to neurons (Gage et al., 2008). These different stages can be identified by a combination of morphological criteria and the expression of molecular markers. For example, aNSCs in the hippocampus are characterized by the expression of SOX2, glial fibrillary acid protein (GFAP), and activated Notch signaling (Lugert et al., 2010), whereas the amplifying neuroblast expresses a combination of PSA-NCAM, doublecortin (DCX), and the mitotic marker Ki-67 (Gage et al., 2008). Doublecortin expression (but not expression of mitotic markers) persists for some time in young terminally differentiated neurons (Brown et al., 2003). Neurons generated during adulthood are functional, integrate into existing circuits, and are proposed to be involved into some specific aspects of behavioral plasticity (van Praag et al., 2002; Gage et al., 2008).
In the present paper, we analyzed the niches of adult neurogenesis in N. furzeri. We found that the neurogenic niches have a similar distribution to that described in detail in zebrafish. In particular, in the telencephalon of both species, there is a clear distinction between a less active pallial and a more active subpallial niche (Adolf et al., 2006; Ganz et al., 2010; Marz et al., 2010). The pallial zone contains slow-cycling mitotically active radial glia positive for S100B, while the subpallial zone contains fast-cycling aNSCs lacking S100B labeling.
Detailed experimental procedures are provided as Data S1.