The role of IGF1 signaling in the development of cerebellar granule cells

During cerebellar development, granule cell progenitors (GCPs) proliferate robustly for a fixed period, promoted by a paracrine mitogenic factor Sonic Hedgehog (Shh) secreted from Purkinje cells (PCs). Dysregulation of Shh signaling leads to uncontrolled GCP proliferation and development of medulloblastoma. Insulin-like growth factor (IGF1) was identified as a critical tumor-promoting factor in a mouse model of the Shh subtype of medulloblastoma. The inactivation of the IGF1 receptor in tumor cells halted tumor progression. This finding was unexpected and suggested that the effect of IGF1 signaling was specific to the tumor cells, which would make it a good therapeutic target. Alternatively, IGF1 signaling could be important for the development of normal GCPs, and thus critical for transformed tumor GCPs. Since understanding the normal function of pathways that become dysregulated and cause tumorigenesis can provide tremendous insights, I sought to determine whether IGF1 was required for normal GCP proliferation. I used a mouse genetic system called Mosaic Analysis with Double Markers (MADM) to determine the functional role of IGF1 signaling in the development of normal GCPs by taking advantage of the unequivocal labeling of mutant and wildtype sibling cells at single-cell resolution. The loss of IGF1R, specifically in GCPs, resulted in a 10-fold reduction in the number of granule cells (GCs) in the cerebellum, indicating that IGF1 is a critical regulator of GC numbers in the cerebellum. Subsequent analyses demonstrated that hindered S-phase entry, and increased cell cycle exit of IGF1R-null cells collectively led to this phenotype. Finally, genetic interaction studies showed that IGF1 signaling prevents GCP cell cycle exit at least partially through suppressing the level of p27kip1, a negative regulator of the cell cycle. In summary, these findings revealed IGF1 as a paracrine factor that positively regulates GCP cell cycle along with Shh, through dampening the level of p27 to prevent precocious cell cycle exit.