It is generally believed that cerebellar granule neurons originate exclusively from granule neuron precursors (GNPs) in the external germinal layer (EGL). Here we identify a rare population of neuronal progenitors in mouse developing cerebellum that expresses Nestin. Although Nestin is widely considered a marker for multipotent stem cells, these Nestin-expressing progenitors (NEPs) are committed to the granule neuron lineage. Unlike conventional GNPs, which reside in the outer EGL and proliferate extensively, NEPs reside in the deep part of the EGL and are quiescent. Expression profiling reveals that NEPs are distinct from GNPs, and in particular, express markedly reduced levels of genes associated with DNA repair. Consistent with this, upon aberrant activation of Sonic hedgehog (Shh) signaling, NEPs exhibit more severe genomic instability and give rise to tumors more efficiently than GNPs. These studies identify a novel progenitor for cerebellar granule neurons and a novel cell of origin for medulloblastoma.

Astrocytes, the most abundant type of glial cells in the brain, play critical roles in supporting neuronal development and brain function. While astrocytes have been frequently detected in brain tumors, including medulloblastoma (MB), their functions in tumorigenesis are not clear. Here we demonstrate that astrocytes are essential components of the MB tumor microenvironment. Tumor-associated astrocytes (TAA) secreted the ligand sonic hedgehog (Shh), which is required for maintaining MB cell proliferation despite the absence of its primary receptor Patched-1 (Ptch1). Shh drove expression of Nestin in MB cells through a Smoothened-dependent, Gli1-independent mechanism. Ablation of TAA dramatically suppressed Nestin expression and blocked tumor growth. These findings demonstrate an indispensable role for astrocytes in MB tumorigenesis and reveal a novel Ptch1-independent Shh pathway involved in MB progression.

The intermediate filament protein Nestin serves as a biomarker for stem cells and has been used to identify subsets of cancer stem-like cells. However, the mechanistic contributions of Nestin to cancer pathogenesis are not understood. Here we report that Nestin binds the hedgehog pathway transcription factor Gli3 to mediate the development of medulloblastomas of the hedgehog subtype. In a mouse model system, Nestin levels increased progressively during medulloblastoma formation resulting in enhanced tumor growth. Conversely, loss of Nestin dramatically inhibited proliferation and promoted differentiation. Mechanistic investigations revealed that the tumor-promoting effects of Nestin were mediated by binding to Gli3, a zinc finger transcription factor that negatively regulates hedgehog signaling. Nestin binding to Gli3 blocked Gli3 phosphorylation and its subsequent proteolytic processing, thereby abrogating its ability to negatively regulate the hedgehog pathway. Our findings show how Nestin drives hedgehog pathway-driven cancers and uncover in Gli3 a therapeutic target to treat these malignancies.