An Adult Zebrafish Brain Atlas To Investigate Shh Mediated Cell-Cell Signaling In Neurogenic Zones

Adult neurogenesis occurs in proliferative zones of the brain that contain neural progenitor cell populations capable of differentiating into specific cell types. However, we remain limited in our understanding of the signals that regulate neural progenitor cell proliferation and differentiation in adults. Recently zebrafish (Danio rerio) have emerged as an excellent model for studying the molecular mechanisms behind adult neurogenesis, because sixteen proliferative zones remain active in the adult brains. Thousands of fluorescent transgenic reporter lines have been generated in zebrafish that reveal gene expression patterns of cell-cell signaling systems, some of which may regulate neurogenesis in these brain regions. Using a new tissue clearing technique and whole brain imaging with fluorescent light sheet microscopy (FLSM) we have generated the first 3-Dimensional atlas of gene expression in an intact adult zebrafish brain. So far we have created a reference brain image and have aligned the expression patterns from three transgenic lines. This work is a preliminary step in the generation of a new, open access brain atlas called the Zebrafish Adult Brain Browser (ZABB). While generating this atlas we focused on documenting the adult brain regions responsive to Sonic Hedgehog (Shh), a cell-cell signaling system known to regulate neurogenesis during embryonic development. We used two Shh-reporter lines to create another atlas comparing reporter transgene expression in whole brain and sectioned tissue to the expression of the Hedgehog (Hh) target gene ptch2 using in situ hybridization. We show that the reporter lines reveal different Hh responsive domains, but together identify fourteen Hh responsive regions in the brain, nine of which are known proliferative zones. Thus, it appears that subsets of both proliferating neural progenitors and non-proliferative cells remain Hh responsive in adult brains. Our data suggests that Hh signaling contributes to the regulation of neural progenitor cells in nine of the sixteen proliferative zones. Uncovering the molecular mechanisms behind adult neurogenesis and forming a greater understanding of adult neural stem cell regulation has the potential to influence the treatment of many neurodegenerative diseases and cancers

Hedgehog Signaling Regulates Neurogenesis in the Larval and Adult Zebrafish Hypothalamus

Neurogenesis is now known to play a role in adult hypothalamic function, yet the cell-cell mechanisms regulating this neurogenesis remain poorly understood. Here, we show that Hedgehog (Hh)/Gli signaling positively regulates hypothalamic neurogenesis in both larval and adult zebrafish and is necessary and sufficient for normal hypothalamic proliferation rates. Hh-responsive radial glia represent a relatively highly proliferative precursor population that gives rise to dopaminergic, serotonergic, and GABAergic neurons. In situ and transgenic reporter analyses revealed substantial heterogeneity in cell-cell signaling within the hypothalamic niche, with slow cycling Nestin-expressing cells residing among distinct and overlapping populations of Sonic Hh (Shh)-expressing, Hh-responsive, Notch-responsive, and Wnt-responsive radial glia. This work shows for the first time that Hh/Gli signaling is a key component of the complex cell-cell signaling environment that regulates hypothalamic neurogenesis throughout life.</p

Innovations present in the primate interneuron repertoire

Primates and rodents, which descended from a common ancestor around 90 million years ago , exhibit profound differences in behaviour and cognitive capacity; the cellular basis for these differences is unknown. Here we use single-nucleus RNA sequencing to profile RNA expression in 188,776 individual interneurons across homologous brain regions from three primates (human, macaque and marmoset), a rodent (mouse) and a weasel (ferret). Homologous interneuron types—which were readily identified by their RNA-expression patterns—varied in abundance and RNA expression among ferrets, mice and primates, but varied less among primates. Only a modest fraction of the genes identified as ‘markers’ of specific interneuron subtypes in any one species had this property in another species. In the primate neocortex, dozens of genes showed spatial expression gradients among interneurons of the same type, which suggests that regional variation in cortical contexts shapes the RNA expression patterns of adult neocortical interneurons. We found that an interneuron type that was previously associated with the mouse hippocampus—the ‘ivy cell’, which has neurogliaform characteristics—has become abundant across the neocortex of humans, macaques and marmosets but not mice or ferrets. We also found a notable subcortical innovation: an abundant striatal interneuron type in primates that had no molecularly homologous counterpart in mice or ferrets. These interneurons expressed a unique combination of genes that encode transcription factors, receptors and neuropeptides and constituted around 30% of striatal interneurons in marmosets and humans