Hedgehog Signaling Regulates HSC Proliferation.
Authors: Merchant Akil , Joseph Giselle , Matsui William 
Published: April 2020

Abstract
Hedgehog (Hh) signaling is essential for normal development and is dysregulated in many cancers. Hh signaling is active in normal bone marrow and the majority of acute myeloid leukemias, however, the precise role of Hh signaling and its positive effector Gli1 in normal or malignant hematopoiesis is not known. We have analyzed the bone marrow of Gli1 null mice to understand the role of this transcription factor in normal hematopoiesis in order to gain insight into its potential role in leukemia. Gli1 null mice develop normally and have normal peripheral blood counts but the bone marrow shows skewing of the c-Kit+Sca1+Lin-neg (KSL) progenitor compartment with increased CD34negKSL long-term HSC (LT-HSC) and decreased 34+KSL short-term HSC (ST-HSC). An analogous difference was observed in the c-Kit+Sca1negLinneg (KL) myeloid progenitor compartment with an increase in FcRγlowCD34+KL common myeloid progenitors (CMP) and decrease in the FcRγhighCD34+KL granulocyte monocyte progenitors (GMP). We speculated that these differences could be due to impaired cell cycle since both the ST-HSC and GMP are more proliferative than LT-HSC and CMP, respectively. Cell cycle analysis by DNA content and BrdU pulse labeling (100mg/kg IP 14 hours prior to analysis) revealed a marked decrease of proliferation in the LT-HSC, ST-HSC, CMP, and GMP compartments of Gli1 null mice. We supported this conclusion by demonstrating that the bone marrow of Gli1 null mice are relatively radio-resistant. Mice exposed to 400 cGy of total body irradiation followed with serial blood counts revealed less severe nadir, but delayed rebound of white blood cells in Gli1 null mice. We further hypothesized that although Gli1 appears to be dispensable for steady-state peripheral hematopoiesis, it might be necessary for rapid proliferation of progenitors needed during stressed hematopoiesis. In brain development, where Hh signaling is much better understood, active Hh signaling is critical for regulating proliferation of neural stem cells and Gli1 activity significantly increases after depletion of neural progenitors with chemotherapy (Bai et al., Development, 2002). To extend this observation to hematopoiesis, we treated Gli1 null mice and wild-type litter-mates with 5-fluorouracil (5-FU) at 100mg/kg and measured serial blood counts. Gli1 null mice had a delayed recovery of total white blood cells and neutrophil counts at 6 days after 5-FU, but this difference normalized by 20 days after treatment. To confirm that this difference was due to impaired proliferation and not increased sensitivity to 5-FU, we treated Gli1 null and wild-type mice with G-CSF (10mcg/kg/day) for three days to stimulate neutrophil proliferation. Confirming our hypothesis, we observed an attenuated neutrophil response in G-CSF stimulated Gli1 null mice. In summary, we have demonstrated that Gli1 loss leads to decreased HSC and myeloid progenitor proliferation, which has important functional consequences for stress hematopoiesis. These data suggest that abnormal Hh activity in leukemia may be important for driving the uncontrolled proliferation of cancer cells. Gli1 null mice were a kind gift from Alexandra Joyner, Memorial Sloan-Kettering Cancer Center