The functions of suppressor of fused (Sufu) in midbrain and hindbrain neurogenesis

Abstract

Suppressor of fused (Sufu) is an intracellular factor which negatively modulates Sonic hedgehog (Shh) signaling through Gli transcription factors. 〖Sufu 〗^(-/-)mice exhibit embryonic lethality at E9.5 with substantial central nervous system (CNS) defects. Mutant studies show that Sufu mediates cerebellum morphogenesis, mid-hindbrain and spinal cord patterning, indicating indispensable role of Sufu in CNS development. To decipher the functions of Sufu, and its downstream regulatory network in embryonic midbrain and hindbrain neurogenesis, tissue specific deletion of the Sufu gene in mutant mouse models were performed in this study.

Pax2-cre and B2-R4-cre were crossed with 〖Sufu 〗^(f/f ) respectively to abolish Sufu in mid-hindbrain (MHB) at around E8.0 (Sufu-cMHB), and hindbrain rhombomere (r) 4 at around E9.0 (Sufu-cr4). Sufu-cMHB exhibited overgrowth of the tectum and cerebellum with impaired morphogenesis. The number of PH3-labeled dividing cells was increased at E12.5. Besides, Shh signaling appeared highly stimulated, as indicated by expanded Shh expression and ventralized MHB neural tube. These results suggest that Sufu attenuates Shh signaling and cell proliferation in MHB. Moreover, the key MHB patterning gene, Fgf8, was ectopically expressed in rostral hindbrain at E10.5, indicating that Sufu may regulate Fgf8 in MHB morphogenesis.

The Sufu-cr4 mutant exhibited an enlarged hindbrain rhombomere 4 (r4). To elucidate the role of Sufu in controlling the size of r4, cell proliferation and differentiation were investigated. In BrdU incorporation analysis, mutant dorsal progenitor domain showed more substantial increase of proliferation than the ventral domain. Predominant accumulation of Sox2+ progenitors, coupled by reduced Tuj+ neuronal differentiation and cell cycle exit was observed at the dorsal region. These results demonstrate novel functions of Sufu for spatial regulation of cell cycle progression and differentiation in the hindbrain. To decode the underlying basis of differentially mediated cell cycle progressions, expression of Shh, Notch and Wnt pathway effectors were examined. Interestingly, in Sufu-cr4, Gli1, Gli2, Hes1 and β-catenin expressions were differentially elevated in different dorso-ventral domains. Furthermore, ablation of Gli2 or β-catenin in the Sufu-cr4 background alleviated Sox2+ progenitors accumulation phenotype of Sufu-cr4, suggesting that Sufu acts as a master regulator of progenitor proliferation and differentiation via dual repression of Gli2 and β-catenin.

Sufu is required to mediate the generation of facial branchiomotor neurons (FBM) in r4. Flat-mount in situ hybridization revealed markedly increased generation of Nkx2.2 and Phox2b positive FBMs in Sufu-cr4 at E11.5. A subpopulation of mutant FBM showed aberrant dorsal segregation near the rhombomere boundaries of r4. Strikingly, these mutant cells preferentially segregated towards the elevated Hes1 expression territories. To test if Gli2 up-regulation causes aberrant FBM phenotypes, Gli2 was deleted in Sufu-cr4, which resulted in reduced FBM generation, recovered ectopic Hes1 expression and dorsal segregation phenotypes. These results suggest that Sufu impedes FBM generation and regulates FBM segregation through Gli2 function.

This study uncovered essential functions of Sufu in embryonic CNS development, revealing its role as a critical modulator for proliferation and differentiation in neural progenitors through repression of Shh-Gli2 and Wnt/β-catenin functions. This study provides the basis for targeting Sufu for promoting neural stem cell regeneration in neurodegeneration diseases.