Abstract
Sonic hedgehog (Shh) is a multifunctional signaling protein governing pattern formation, proliferation and cell survival during embryogenesis. In the adult brain, Shh has neurotrophic function and is implicated in hippocampal neurogenesis but the cellular source of Shh in the hippocampus remains ill defined. Here, we utilize a gene expression tracer allele of Shh (Shh-nlacZ) which allowed the identification of a subpopulation of hilar neurons known as mossy cells (MCs) as a prominent and dynamic source of Shh within the dentate gyrus. AAV-Cre mediated ablation of Shh in the adult dentate gyrus led to a marked degeneration of MCs. Conversely, chemical stimulation of hippocampal neurons using the epileptogenic agent kainic acid (KA) increased the number of Shh+ MCs indicating that the expression of Shh by MCs confers a survival advantage during the response to excitotoxic insults. In addition, ablation of Shh in the adult dentate gyrus led to increased neural precursor cell proliferation and their migration into the subgranular cell layer demonstrating that MCs-generated Shh is a key modulator of hippocampal neurogenesis.
Highlights
Sonic hedgehog (Shh) is a multifunctional signaling protein governing pattern formation, proliferation and cell survival during embryogenesis
Our results suggest that Shh expression in adult mossy cells (MCs) serves as a neuro-protectant for MCs, as a chemo attractant for immature neuronal precursor cells that ectopically migrate to the hilus to become CR+ cells during induced excitotoxicity, and as an inhibitor of neuronal cell fates that home to the granular cell layer
Shh and nucleus-targeted LacZ is transcribed into a bi-cistronic mRNA from the endogenous Shh locus such that all cells that express Shh are marked by nuclear localized betaGal allowing sensitive chromogenic and fluorescent immunohistochemical analysis of Shh expression with single cell resolution
Summary
Sonic hedgehog (Shh) is a multifunctional signaling protein governing pattern formation, proliferation and cell survival during embryogenesis. The deletion of Shh from these CR+ cells was associated with a significant decrease in proliferation and the number neuronal stem cells (NSCs)[6] Whether these neurons express Shh in the adult hippocampus has not been studied. We re-examined the expression of Shh within the hippocampus using a sensitive Shh gene expression tracer allele which marks nuclei of Shh expressing cells by nuclear targeted lacZ and allows selective identification of cells in which the Shh locus is transcriptionally active This reporter was used previously to discover that mesencephalic dopamine neurons are a significant source of Shh throughout adulthood in the forebrain[13]
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