Shh-mediated centrosomal recruitment of PKA promotes symmetric proliferative neuroepithelial cell division

Abstract

Tight control of the balance between self-expanding symmetric and self-renewing asymmetric neural progenitor divisions is crucial to regulate the number of cells in the developing central nervous system. We recently demonstrated that Sonic hedgehog (Shh) signalling is required for the expansion of motor neuron progenitors by maintaining symmetric divisions. Here we show that activation of Shh/Gli signalling in dividing neuroepithelial cells controls the symmetric recruitment of PKA to the centrosomes that nucleate the mitotic spindle, maintaining symmetric proliferative divisions. Notably, Shh signalling upregulates the expression of pericentrin, which is required to dock PKA to the centrosomes, which in turn exerts a positive feedback onto Shh signalling. Thus, by controlling centrosomal protein assembly, we propose that Shh signalling overcomes the intrinsic asymmetry at the centrosome during neuroepithelial cell division, thereby promoting self-expanding symmetric divisions and the expansion of the progenitor pool.

Cell biological mechanisms regulating chick neurogenesis

“…Localisation of PKA to the centrosome is thought to block the PKA kinase activity, shifting the balance towards more activated Gli (Barzi et al, 2010, Saade et al, 2017. In a recent study, PKA was found symmetrically localised to both mitotic centrosomes in the early chick neuroepithelium where divisions are almost exclusively proliferative (Saade et al, 2017). At later stages, when asymmetric divisions predominate, PKA appeared asymmetrically localised, with high levels in only one centrosome (Fig.…”

“…Protein kinase A (PKA) also plays a role in regulating Gli activity by phosphorylating Gli proteins at 6 sites, rendering them inactive (Niewiadomski et al, 2014). Localisation of PKA to the centrosome is thought to block the PKA kinase activity, shifting the balance towards more activated Gli (Barzi et al, 2010, Saade et al, 2017. In a recent study, PKA was found symmetrically localised to both mitotic centrosomes in the early chick neuroepithelium where divisions are almost exclusively proliferative (Saade et al, 2017).…”

“…These findings therefore uncover a correlation between Shh activity and centrosome maturation that results in symmetric PKA centrosome docking, which in turn promotes Shh signalling and so neural progenitor proliferation. A key question for the future is how Shh signalling promotes expression of pericentin and other centrosomal genes, which appear not to be direct Gli targets (Saade et al, 2017). As addressed below, there may also be mechanisms to ensure Shh signalling loss as the centrosome matures in the differentiating neuron.…”

“…These inherent centrosomal differences must clearly be overcome for cells to undergo symmetric divisions that generate two progenitors. Recent findings in the chick embryo have now begun to reveal how this is achieved by signalling pathways which promote neural progenitor proliferation (Saade et al, 2017, Tozer et al, 2017.…”

“…Notably, neuroepithelial cells with activated versely, down-regulation of Shh by a dominant-negative form of the receptor Patched 1 resulted in reduced pericentrin expression and asymmetric PKA centrosomal localisation. Significantly, Shh signalling and pericentrin-mediated PKA docking are proposed to operate in a positive feedback loop: reduction in centrosome localised PKA (by mis-expression of a mutant PKA construct which effectively undocks PKA), reduced Gli transcriptional activation and induced the expression of the Tis21-Luc reporter, a marker of neurogenic progenitors and neurons (Saade et al, 2017). These findings therefore uncover a correlation between Shh activity and centrosome maturation that results in symmetric PKA centrosome docking, which in turn promotes Shh signalling and so neural progenitor proliferation.…”