Coordinated d-cyclin/Foxd1 activation drives mitogenic activity of the Sonic Hedgehog signaling pathway

Abstract

Sonic Hedgehog (Shh) signaling plays key regulatory roles in embryonic development and postnatal homeostasis and repair. Modulation of the Shh pathway is known to cause malformations and malignancies associated with dysregulated tissue growth. However, our understanding of the molecular mechanisms by which Shh regulates cellular proliferation is incomplete. Here, using mouse embryonic fibroblasts, we demonstrate that the Forkhead box gene Foxd1 is transcriptionally regulated by canonical Shh signaling and required for downstream proliferative activity. We show that Foxd1 deletion abrogates the proliferative response to SHH ligand while FOXD1 overexpression alone is sufficient to induce cellular proliferation. The proliferative response to both SHH ligand and FOXD1 overexpression was blocked by pharmacologic inhibition of cyclin-dependent kinase signaling. Time-course experiments revealed that Shh pathway activation of Foxd1 is followed by downregulation of Cdkn1c, which encodes a cyclin-dependent kinase inhibitor. Consistent with a direct transcriptional regulation mechanism, we found that FOXD1 reduces reporter activity of a Fox enhancer sequence in the second intron of Cdkn1c. Supporting the applicability of these findings to specific biological contexts, we show that Shh regulation of Foxd1 and Cdkn1c is recapitulated in cranial neural crest cells and provide evidence that this mechanism is operational during upper lip morphogenesis. These results reveal a novel Shh-Foxd1-Cdkn1c regulatory circuit that drives the mitogenic action of Shh signaling and may have broad implications in development and disease.

An autoregulatory cell cycle timer integrates growth and specification in chick wing digit development

“…Therefore, the fact that Shh signalling controls the expression of Cyclin D2 and p27 kip1 with different temporal dynamics, provides a robust auto-regulatory feedback loop for modulating polarising region proliferation in the chick wing (Figure 8a). Our findings are consistent with many studies demonstrating that Shh signalling can either stimulate proliferation via D cyclins (Kenney and Rowitch, 2000; Komada et al, 2013; Fink et al, 2018; Lobjois et al, 2004; Mill et al, 2005; Seifert et al, 2010) or inhibit proliferation via D cyclin inhibitors (Shkumatava and Neumann, 2005; Fu et al, 2017; Parathath et al, 2010). It could be that Shh signalling fulfils both functions in most tissues, but at different times, and this could be the focus of future endeavours.…”

Up-regulation of FOXD1 by YAP alleviates senescence and osteoarthritis

“…As a member of the forkhead box family of transcription factors, FOXD1 is known to regulate kidney development during organogenesis [60,61]. Recently, FOXD1 has been shown to promote cell proliferation by targeting the sonic hedgehog pathway and cyclin-dependent kinase inhibitors [62,63]. FOXD1 also facilitates the reprogramming of mouse embryonic fibroblasts (MEFs) into induced pluripotent stem cells (iPSCs) [64].…”

Development of the basal hypothalamus through anisotropic growth

“…Key to this growth loop is the ability of Shh to non‐autonomously induce Rx , for Rx to autonomously induce Shh , and for Shh to subsequently autonomously down‐regulate Rx for cells to realise the anterior growth programme. Potentially, Shh‐Rx interact additionally with Foxd1 and/or Six3: each interacts with Shh and each promotes proliferation …”

“…Key to this growth loop is the ability of Shh to non-autonomously induce Rx, for Rx to autonomously induce Shh, and for Shh to subsequently autonomously down-regulate Rx for cells to realise the anterior growth programme. Potentially, Shh-Rx interact additionally with Foxd1 and/or Six3: each interacts with Shh and each promotes proliferation 33,[46][47][48]. Having acted with Rx to select anterior progenitors, in a subsequent step, Shh appears to up-regulate p57 kip28 and components of the Notch pathway49,50 to promote a neurogenic differentiation programme (Figure 2B,C).…”