Sonic Hedgehog and its GLI transcriptional effectors control foliation complexity during cerebellar morphogenesis by promoting granule cell precursor proliferation. Here, we reveal a novel contribution of Sonic Hedgehog-GLI signaling to cerebellar patterning and cell differentiation by generating mice with targeted deletion of Suppressor of Fused (SuFu), a regulator of Sonic Hedgehog signaling, in the mid-hindbrain. Postnatal SuFu-deficient mice exhibit impaired motor coordination and severe cerebellar mispatterning. SuFu conditional knock-out embryos display abnormal mid-hindbrain morphology associated with misexpression of Fgf8, and delayed differentiation and abnormal migration of major cerebellar cell types. Sonic Hedgehog is ectopically expressed in the external granule layer and Hedgehog signaling is upregulated. While expression of full-length GLI transcriptional activators downstream of Hedgehogs is markedly reduced, a processed form of GLI3, a transcriptional repressor, is essentially lost. Genetic expression of a Gli3 allele encoding GLI3 repressor in SuFu-deficient mice largely rescues abnormal cerebellar patterning and cell differentiation observed in mice with SuFu deficiency alone. Together, our data demonstrate that SuFu controls cerebellar patterning and cell differentiation in a GLI3 repressordependent manner.

Together, our results show that SuFu promotes cerebellar radial precursor differentiation to neurons. SuFu function is mediated in part by GLI3R and down-regulation of Fgf15 expression. Developmental Dynamics 247:156-169, 2018. © 2017 Wiley Periodicals, Inc.

Cerebellar granule cell (GC) development relies on precise regulation of sonic hedgehog (Shh)-Gli signalling activity, failure of which is associated with motor disorders and medulloblastoma. Mutations in the pathway regulator suppressor of fused (Sufu), which modulates Gli activators and repressors, are linked to cerebellar dysfunction and tumourigenesis. The mechanism by which Sufu calibrates Shh signalling in GCs is unknown. Math1-Cre-mediated deletion of Sufu in mouse GC progenitors (GCPs) demonstrated that Sufu restricts GCP proliferation and promotes cell cycle exit, by promoting expression of Gli3R and suppressing Gli2 levels. Sufu is also required to promote a high threshold of pathway activity in GCPs. Remarkably, central cerebellar lobules are more deleteriously impacted by Sufu deletion, but are less sensitive to downstream genetic manipulations to reduce Gli2 expression or overexpress a Gli3R mimic, compared with anterior lobules. Transcriptome sequencing uncovered new Sufu targets, especially Fgf8, which is upregulated in Sufu-mutant GCPs. We demonstrate that Fgf8 is necessary and sufficient to drive Sufumutant GCP proliferation. This study reveals new insights into the spatial and temporal regulation of cerebellar Shh-Gli signalling, while uncovering new targets, such as Fgf8.