Duration of Shh signaling contributes to mDA neuron diversity

Abstract

Sonic hedgehog (Shh) signaling is critical for various developmental processes including specification of the midbrain dopamine (mDA) neurons in the ventral mesencephalon (vMes). While the timing of Shh and its response gene Gli1 segregates mDA neurons, their overall lineage contribution to mDA neurons heavily overlaps. Here, we demonstrate that the same set of mDA neuron progenitors sequentially respond to Shh signaling (Gli1 expression), induce Shh expression, and then turn off Shh responsiveness. Thus, at any given developmental stage, cells rarely co-express Shh and Gli1. Using ShhCre:GFP mice to delete the Smoothened receptor in the Shh pathway, we demonstrate that the loss of Shh signaling in Shh expressing cells results in a transient increase in proliferation and subsequent depletion of mDA neuron progenitors in the posterior vMes due to the facilitated cell cycle exit. Moreover, the change in duration of Shh signaling in vMes progenitors altered the timing of the contribution to the ventral tegmental area (VTA) and the substantia nigra pars compacta (SNc) mDA neurons. Taken together, our investigation on the relationship between the Shh-secreting and -responding cells revealed an intricate regulation of induction and cessation of Shh signaling that influences the distribution of mDA neurons in the VTA and SNc.

Nicotinamide N-methyltransferase expression in SH-SY5Y neuroblastoma and N27 mesencephalic neurones induces changes in cell morphology via ephrin-B2 and Akt signalling

“…Substantia nigra dopaminergic neurones are derived from the mesencephalon, 32 hence N27 cells closely mimic dopaminergic neurones of the substantia nigra. Wild-type N27 cells expressed no NNMT mRNA (Supplementary Figure S3).…”

Primary cilia are critical for Sonic hedgehog-mediated dopaminergic neurogenesis in the embryonic midbrain

“…The fact that mature mDA neurons are generated in Ift88 cko mutants suggests that cilia are not essential for mediating Wnt or Fgf signaling in mDA neuronal progenitors after E8.5. However, the reductions in the expression levels of Wnt growth factors and Axin2 in Ift88 cko mutants are likely a direct consequence of the loss of primary cilia, rather than an indirect effect of the reduction in Shh signaling, since it has been reported that Shh signaling regulates Wnt1 expression negatively (Hayes et al, 2013; Tang et al, 2013). Moreover, we did not observe an obvious reduction in Wnt1 expression in the Gli2/3 cko mutants.…”

Establishing diversity in the dopaminergic system

“…The Shh expressing domain expands further until E10.5; accordingly the adjacent Gli1 expressing domain moves further laterally. After E10.5, Shh is downregulated in ventral midline progenitors, but these medial progenitors remain unresponsive to SHH signaling and remain negative for Gli1 and other components of the canonical SHH signaling pathway [64–68]. The Shh expressing domain is eventually considerably broader than the MbDN pogenitor domain (Fig.…”

“…The defects in MbDN progenitors in mice, in which SHH signaling is conditionally inactivated at different developmental time points, are consistent with the fate-mapping results of SHHresponsive MbDN progenitors and confirm that SHH signaling is required early during MbDN progenitor development: Inactivation of SHH signaling after E10.5 does not affect the generation of MbDN progenitors while inactivation after E9.0 results in an only transient and mild defect in MbDN proliferation and neurogenesis. Inactivation of SHH signaling after E8.5 leads to a strong reduction in the size of the MbDN progenitor domain and in the number of mature MbDNs, but not to their complete absence [62,67,68,71,72]. Only complete inactivation of SHH signaling results in the loss of the entire MbDN progenitor population.…”