IGF1‐mTORC1 Signaling Drives Intestinal Epithelial Regeneration After Irradiation Injury
Authors: Abushukur Yasmine , Bohin Natacha , McGowan Kevin P , Keeley Theresa M , Carlson Elizabeth A , Samuelson Linda C 
Published: June 2021

Intestinal stem cells play a crucial role in maintaining intestinal tissue throughout lifespan by proliferating and differentiating to generate the various mature epithelial cell types. When stem cells are lost due to intestinal injury, the epithelium has a remarkable capacity for repair. In this process, facultative stem cells (FSCs) within the intestinal crypt mobilize to replace lost stem cells and repair the damaged tissue. In this study, we investigated the mechanism driving intestinal regeneration after injury induced by 12 Gy γ‐irradiation. Analysis of various growth factor responses post‐irradiation demonstrated a 6‐fold increase in insulin‐like growth factor 1 (IGF1) mRNA abundance during the regenerative phase. To test IGF1 function in intestinal regeneration, we blocked signaling by treating irradiated mice with the IGF1 receptor inhibitor BMS‐754807. BMS‐treated mice showed enhanced weight loss after irradiation compared to vehicle‐treated irradiated controls, with histological analysis of cellular proliferation demonstrating reduced crypt regeneration at 3 days post‐irradiation. To investigate the mechanism of IGF1 action, we investigated the IGF1 downstream effector complex, mammalian target of rapamycin complex 1 (mTORC1). Western blot analysis of intestinal tissue post‐irradiation showed that mTORC1 was activated during the regenerative response. Pharmacological inhibition of mTORC1 activity by treating irradiated mice with rapamycin mirrored the effect of IGF1 inhibition, with enhanced weight loss and impaired crypt regeneration. To identify the key cells exhibiting mTORC1 activity, we used a genetic approach to delete mTORC1 in the intestinal epithelium using Villin‐CreERT2;RaptorF/F mice. Mice were treated with tamoxifen to induce Cre recombinase‐mediated deletion of Raptor, which encodes a key subunit of the mTORC1 complex. The Raptor‐deleted mice showed enhanced weight loss and impaired crypt regeneration post‐irradiation, suggesting that mTORC1 epithelial cell signaling was required for crypt regeneration. We tested whether an FSC population marked by Bmi1 required mTORC1 signaling to be activated during regeneration. After tamoxifen treatment, Bmi1‐CreERT2;Rosa‐lacZ mice were irradiated and treated with rapamycin or vehicle. Analysis of lacZ lineage‐tracing by staining for beta‐galactosidase activity showed that mTORC1 inhibition reduced FSC activation post‐irradiation. These findings suggest that intestinal injury induces IGF1, which signals to intestinal epithelial cells to induce FSC mobilization and intestinal epithelial regeneration via mTORC1 activation.Support or Funding InformationYA was supported by the APS Undergraduate Summer Research Fellowship, NB was supported by the CMB program, a Rackham research grant and the Benard L Maas Fellowship, and KM was supported by the Training Program in Organogenesis T32‐HD007505. The research was funded by NIH R01‐DK118023 awarded to LS.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.