Suppressor of fused controls cerebellum granule cell proliferation by suppressing Fgf8 and spatially regulating Gli proteins

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 Sufu-mutant 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.

Suppressor of fused controls cerebellar neuronal differentiation in a manner modulated by GLI3 repressor and Fgf15

BACKGROUND

Deficiency of Suppressor of Fused (SuFu), an intracellular mediator of Hedgehog signaling, in the murine mid-hindbrain disrupts cerebellar morphogenesis and cell differentiation in a manner that is rescued by constitutive expression of GLI3 transcriptional repressor (GLI3R). Here, we determined SuFu functions in cerebellar radial precursors following the stage of mid-hindbrain specification using a Blbp-Cre transgene.

RESULTS

SuFu-deficient cerebella were severely dysplastic, and characterized by laminar disorganization, and delayed differentiation of ventricular zone-derived precursors. In vitro analysis of cerebellar precursors isolated from control and mutant mice demonstrated an increased proportion of radial glial precursors vs. Tuj1-positive neurons in mutant cultures. Abnormal cell differentiation in SuFu-deficient precursors was rescued by a constitutively expressed GLI3R knock-in allele, albeit with variable penetrance. Using RNA expression analysis in control and SuFu-deficient cerebellar anlage, we identified up-regulation of Fgf15 in mutant tissue. Strikingly, exogenous hFGF19, a mFGF15 ortholog, inhibited neuronal differentiation in cultures of wild-type cerebellar precursors. Moreover, siRNA-mediated knockdown of Fgf15 in SuFu-deficient cerebellar precursors rescued their delayed differentiation to neurons.

CONCLUSIONS

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.