Forkhead Transcription Factor FoxM1 Regulates Mitotic Entry and Prevents Spindle Defects in Cerebellar Granule Neuron Precursors

Abstract

The forkhead transcription factor FoxM1 has been reported to regulate, variously, proliferation and/or spindle formation during the G₂/M transition of the cell cycle. Here we define specific functions of FoxM1 during brain development by the investigation of FoxM1 loss-of-function mutations in the context of Sonic hedgehog (Shh)-induced neuroproliferation in cerebellar granule neuron precursors (CGNP). We show that FoxM1 is expressed in the cerebellar anlagen as well as in postnatal proliferating CGNP and that it is upregulated in response to activated Shh signaling. To determine the requirements for FoxM1 function, we used transgenic mice carrying conventional null alleles or conditionally targeted alleles in conjunction with specific Cre recombinase expression in CGNP or early neural precursors driven by Math1 or Nestin enhancers. Although the overall cerebellar morphology was grossly normal, we observed that the entry into mitosis was postponed both in vivo and in Shh-treated CGNP cultures. Cell cycle analysis and immunohistochemistry with antibodies against phosphorylated histone H3 indicated a significant delay in the G₂/M transition. Consistent with this, FoxM1-deficient CGNP showed decreased levels of the cyclin B1 and Cdc25b proteins. Furthermore, the loss of FoxM1 resulted in spindle defects and centrosome amplification. These findings indicate that the functions of FoxM1 in Shh-induced neuroproliferation are restricted to the regulation of the G₂/M transition in CGNP, most probably through transcriptional effects on target genes such as those coding for B-type cyclins.

SUPPLEMENTAL MATERIAL

We thank Jason Ling, Dongin Yuk, and Sovann Kaing for excellent technical assistance. We also thank the late Robert Costa (University of Illinois, Chicago, IL) for contributing FoxM1^Fl/Fl mice, Jane Johnson (University of Texas, Dallas, TX) for providing the Math1 plasmid, and Fabienne Pituello (Université P. Sabatier, Toulouse, France) for providing the Cdc25b plasmid.

U.S. is a scholar of the Mildred-Scheel-Stiftung für Krebsforschung. This work was supported by grants (to D.H.R.) from the NINDS (NS 047527), the James S. McDonnell Foundation, and the Pediatric Brain Tumor Foundation of the United States.