Gli2 trafficking links Hedgehog-dependent activation of Smoothened in the primary cilium to transcriptional activation in the nucleus

Sonic hedgehog (Shh) is a morphogenic protein that operates through the Gli transcription factor-dependent canonical pathway to orchestrate normal development of many tissues. Because aberrant levels of Gli activity lead to a wide spectrum of diseases ranging from neurodevelopmental defects to cancer, understanding the regulatory mechanisms of Shh canonical pathway is paramount. During early stages of spinal cord development, Shh specifies neural progenitors through the canonical signaling. Despite persistence of Shh as spinal cord development progresses, Gli activity is switched off by unknown mechanisms. In this study we find that Shh inverts its action on Gli during development. Strikingly, Shh decreases Gli signaling in the embryonic spinal cord by an electrical activity- and cAMP-dependent protein kinase-mediated pathway. The inhibition of Gli activity by Shh operates at multiple levels. Shh promotes cytosolic over nuclear localization of Gli2, induces Gli2 and Gli3 processing into repressor forms, and activates cAMP-responsive element binding protein that in turn represses gli1 transcription. The regulatory mechanisms identified in this study likely operate with different spatiotemporal resolution and ensure effective down-regulation of the canonical Shh signaling as spinal cord development progresses. The developmentally regulated intercalation of electrical activity in the Shh pathway may represent a paradigm for switching from canonical to noncanonical roles of developmental cues during neuronal differentiation and maturation.

Section: Shh Facilitates Processing Of Gli2/3 In the Embryonic Spinalmentioning

confidence: 99%

Inversion of Sonic hedgehog action on its canonical pathway by electrical activity

Belgacem

Borodinsky

2015

“…Activated Smo accumulates within the primary cilium (10,11) and induces the dissociation of Sufu-Gli complexes (4,5), abrogating Gli2/3R formation and promoting the cilium-dependent conversion of full-length Gli proteins (Gli2/ 3FL) into transcriptionally active forms (Gli2/3A) (12,13). Hh signaling coincides with the steady-state accrual of Gli2/3 at the distal tip of the primary cilium (8,14), and Gli2/3A translocate to the nucleus to drive the expression of Ptch1, the constitutively active factor Gli1, and other Hh target genes (1,15).…”

mentioning

confidence: 99%

Arhgap36-dependent activation of Gli transcription factors

Rack¹,

Ni²,

Payumo³

et al. 2014

Hedgehog (Hh) pathway activation and Gli-dependent transcription play critical roles in embryonic patterning, tissue homeostasis, and tumorigenesis. By conducting a genome-scale cDNA overexpression screen, we have identified the Rho GAP family member Arhgap36 as a positive regulator of the Hh pathway in vitro and in vivo. Arhgap36 acts in a Smoothened (Smo)-independent manner to inhibit Gli repressor formation and to promote the activation of full-length Gli proteins. Arhgap36 concurrently induces the accumulation of Gli proteins in the primary cilium, and its ability to induce Gli-dependent transcription requires kinesin family member 3a and intraflagellar transport protein 88, proteins that are essential for ciliogenesis. Arhgap36 also functionally and biochemically interacts with Suppressor of Fused. Transcriptional profiling further reveals that Arhgap36 is overexpressed in murine medulloblastomas that acquire resistance to chemical Smo inhibitors and that ARHGAP36 isoforms capable of Gli activation are upregulated in a subset of human medulloblastomas. Our findings reveal a new mechanism of Gli transcription factor activation and implicate ARHGAP36 dysregulation in the onset and/or progression of GLI-dependent cancers.

“…Smo undergoes phosphorylation by multiple kinases that promote its active conformation and cell surface (Drosophila)/primary cilium (vertebrates) accumulation (11,(14)(15)(16)(17)(18)(19)(20). Smo-mediated intracellular signal transduction abrogates Ci/Gli processing into Ci R /Gli R and converts accumulated full-length Ci/Gli into Ci A /Gli A by dissociating Ci/Gli from Cos2/Kif7 and Sufu (8,(21)(22)(23)(24)(25)(26)(27). The Drosophila Ser/Thr kinase Fused (Fu) is required to antagonize Cos2-and Sufu-mediated inhibition of Ci (28)(29)(30), but its mammalian counterpart remains to be identified.…”

mentioning

confidence: 99%

Hedgehog-induced phosphorylation by CK1 sustains the activity of Ci/Gli activator

Shi¹,

Li²,

et al. 2014

Hedgehog (Hh) signaling governs many developmental processes by regulating the balance between the repressor (Ci R /Gli R ) and activator (Ci A /Gli A ) forms of Cubitus interruptus (Ci)/gliomaassociated oncogene homolog (Gli) transcription factors. Although much is known about how Ci R /Gli R is controlled, the regulation of Ci A /Gli A remains poorly understood. Here we demonstrate that Casein kinase 1 (CK1) sustains Hh signaling downstream of Costal2 and Suppressor of fused (Sufu) by protecting Ci A from premature degradation. We show that Hh stimulates Ci phosphorylation by CK1 at multiple Ser/Thr-rich degrons to inhibit its recognition by the Hh-induced MATH and BTB domain containing protein (HIB), a substrate receptor for the Cullin 3 family of E3 ubiquitin ligases. In Hh-receiving cells, reduction of CK1 activity accelerated HIB-mediated degradation of Ci A , leading to premature loss of pathway activity. We also provide evidence that Gli A is regulated by CK1 in a similar fashion and that CK1 acts downstream of Sufu to promote Sonic hedgehog signaling. Taken together, our study not only reveals an unanticipated and conserved mechanism by which phosphorylation of Ci/Gli positively regulates Hh signaling but also provides the first evidence, to our knowledge, that substrate recognition by the Cullin 3 family of E3 ubiquitin ligases is negatively regulated by a kinase.