Intrinsic Regulation of Ileal Epithelial Cell Proliferation by the Bacterial Peptidoglycan Sensor Nod2

Polymorphisms in the gene encoding for the cytosolic peptidoglycan receptor NOD2 are known to confer significant risk of developing ileal Crohn’s disease.  While much of the research into this gene’s function has been focused on the hematopoietic compartment, NOD2 is also known to be highly expressed by intestinal epithelial cells (IECs), and recent evidence suggests that it plays a significant role in epithelial proliferation and maintenance of barrier integrity.  To examine this further, we used the Cre-loxP system to generate an IEC-specific Nod2 knockout mouse (Nod2^ΔIEC), in which we induced small intestinal inflammation by ip. injection with 10mg/kg of doxorubicin hydrocholoride.  Surprisingly, Nod2^ΔIEC  mice were significantly protected from doxorubicin-ileitis as compared to Nod2-sufficient littermate controls at all time points as assessed by body weight changes and examination of H&E-stained microscopic sections.  As doxorubicin is toxic to rapidly dividing cells, we hypothesized that Nod2^ΔIEC small intestinal crypts might have a lower proliferation rate than those from littermate controls, thus resulting in decreased levels of crypt cell death.  In support of this, we found that small intestinal crypts in naïve Nod2^ΔIEC mice display significantly lower levels of numbers of proliferating cells than their Nod2-sufficient littermates as measured by immunohistochemical staining for the presence of the proliferation marker Ki-67 and the incorporation of the exogenously added thymidine analog BrdU.   Taken together, these results suggest that the proliferation rate of small intestinal epithelial cells can be modulated by NOD2 in a yet to be determined mechanism.