Regulation of NHE3 by the mTOR/autophagy pathways in diarrheal diseases

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DESCRIPTION (provided by applicant): Chronic diarrhea and weight loss with episodic profound acute diarrhea frequently occur in organ transplant recipients related to the use of immunosuppressants. Severe forms of diarrhea require multiple hospitalizations, which are costly, affect patients' quality of life and significantly increase the risk of graft loss and patint death. Sirolimus (rapamycin) is frequently used as an immunosuppressant in organ transplantations. The incidence of diarrhea related to rapamycin use varies from 25% to 42%. Among the various causes of diarrhea (e.g., infection), a significant portion of organ recipients experience diarrhea associated with the use of rapamycin without a clear cause. It is therefore of paramount importance to understand the mechanisms underlying rapamycin-induced diarrhea. Rapamycin's action is mediated by inhibiting the mammalian Target of Rapamycin pathway (mTOR). Inhibition of mTOR by rapamycin stimulates autophagy activity. Thus, the mTOR and autophagy pathways are strong candidates for mediating rapamycin- induced diarrhea, but the exact mechanism of mTOR and/or autophagy in diarrheal disease remains entirely unclear. The brush border Na+/H+ exchanger 3 (NHE3), primarily residing at the apical membrane in the intestine, is known to be responsible for the majority of Na and water absorption. Deletion of NHE3 is sufficient to cause diarrhea. We thus hypothesize that rapamycin treatment alters the mTOR and autophagy pathways, which in turn impinge on NHE3, resulting in diarrhea. Indeed, our preliminary data demonstrated a direct role of suppression of NHE3 by rapamycin in diarrheal patients as well as in animals. The objective of this proposal is to use genetically modified mouse models to further pinpoint if NHE3 regulation by rapamyicn is mediated by the mTOR and autophagy pathways in intestinal epithelial cells. We will use the intestinal epithelial cell- specific mTOR and Atg7 knockout (mTOR-/- and Atg7-/-) mice, with which we will ask if NHE3 expression is altered in mTOR-/- and Atg7-/- mice (Aim IA and IIA) and if rapamycin loses its effect in mTOR-/- and Atg7-/- mice (Aim IB and IIB). The results from this proposal could provide new insights into the roles of the mTOR and autophagy pathways in regulation of the apical membrane transporter NHE3, and may aid in the development of new therapeutic strategies for diarrheal diseases.