The Role Of ERK1/2(MAPK3/1)in Mouse Intestine Development,Homeostasis And Regeneration

The epithelium of mammalian intestine undergoes extremely rapid turnover which is 3 to 5 days in mouse and human.Intestinal stem cells reside near the bottom of the intestinal crypt actively produce progenies through differentiation and self-renew process to maintain the intestinal homeostasis.Once leave the stem cell niche,the stem cells differentiate to rapidly dividing TA progenitor cells through a neutral competition mechanism.The progenitors proliferate and differentiate into terminal differentiation cells such as Paneth cell,goblet cell,enteroendocrine cell,enterocyte and so on.Intestinal homeostasis is tightly controlled by various signals from paneth cells and mesenchymal cells,including four well-characterized signaling pathways,EGF,Notch,Wnt and BMP.Wnt is the key pathway to maintain stem cell fate and drive proliferation of stem and TA cells.BMP signal provides signals to antagonize Wnt signaling and results in differentiation.Disruption of these signals will lead to tumor.Aberrant activation of Wnt signaling causes adenoma,and BMP inactivation leads to juvenile polyposis syndrome(JPS),an hereditary diseases with a risk of the gastrointestinal tract tumor formation.EGF signal exerts strong mitogenic effects on stem and TA cells through Ras/Raf/Mek/Erk kinase cascade.In addition,EGF is an indispensable growth factor for intestinal stem cell culture in vitro.EGFR-/-mouse die after birth suffering from impaired epithelial development in several organs,including skin,lung and gastrointestinal tract.However,the role of its downstream effector in intestine development and homeostasis is not fully understood.As the most important downstream effector of EGF,the role of Erk1/2 in intestine epithelial cells of the intestine remains obscure.We generated intestinal epithelial cell specific knockout mice in Erk1/2 loci through crossing Erk1-/-Erk2flox/flox mice with villin-cre transgenic mice.The conditional knockout mice were growth retarded and died early before adulthood.Surprisingly,histological analysis found abnormally branched villi formed hamartoma which exhibit ectopic epithelial proliferation.The number of Paneth cells was increased but that of goblet cells was decreased,suggesting Erk1/2 played critical role in differentiation of progenitors into secretory lineage.Intestine specific-Erk1/2 deletion also led to broad impact on signaling pathways tightly controlling intestinal homeostasis:Wnt and Notch signal were activated and BMP,Hedgehog and PDGF signaling were suppressed.To explore the mechanism which caused the hamartoma in intestine specific-Erk1/2 mutant mice,we found that AKT-mTOR,the master regulatory signaling of hamartoma syndromes was significantly up-regulated.Co-deletion of Erk1/2 and Tscl in mice intestine showed more severe defects than Erk1/2 knockout.Inhibition of mTOR signal through Rapamycin in intestine specific-Erkl/2 knockout mice,the mice surviving rate was dramatically increased and the hamartoma formation was significantly suppressed.In addition,the differentiation of paneth and goblet cells was rescued.These results further confirmed that AKT-mTOR hyperactivation after Erk1/2 deletion was mainly involved in intestine development and hamartoma formation in intestine specific-Erk1/2 knockout mice.EGF/Erk 'signaling pathway is essential for cell proliferation and homeostasis in adulthood.To determine the physiological role of Erkl/2 in adult intestinal epithelial cells,an intestinal specific inducible knockout strain was generated through crossing Lgr5-EGFP-creERT2 mouse and Erk1/2 conditional knockout mouse.After intraperitoneal injection of tamoxifen,Lgr5 stem cells with deletion of Erk1/2 disappeared,suggesting Erkl/2 is essential for the maintenance of Lgr5-positive stem cells.However,intestinal homeostasis remained normal.It was consistent with previous report that depletion of Lgr5 stem cells in adult mice had no effect on intestine homeostasis but essential for regeneration after radiation-induced cell death.Deletion of Erk1/2 in Lgr5-positive stem cells in mice led to mortality due to the complete abolishment of regeneration capacity in intestinal epithelial cells after X-Ray radiation.In conclusion,our study revealed the pivotal role of Erk1/2 on intestinal development,homeostasis and regeneration in mice.Deletion of Erk1/2 in intestinal epithelial cells led to hyperproliferation of progenitor cells with inhibition of their differentiation and formation of hamartoma.In this study,we also determined the compensatory activation of tumor-prone Akt-mTOR pathway upon Erk1/2 deletion.As ERK inhibitors are promising potential anti-cancer drugs,the potential tumor-promoting effect of ERK inhibition may exist through our study.Combination of ERK and mTOR inhibitors will be a good strategy for treatment of intestinal tumors.