PROJECT SUMMARY Diarrheal disease causes close to one million deaths in children under five each year. Although its incidence is much lower in the more affluent nations, diarrhea remains one of the two most common visits to pediatric emergency rooms and is also common among the institutionalized elderly. NHE3 is a major sodium transporter in the brush border membrane of the small intestine and proximal colon. Abnormal NHE3 expression and function are associated with diarrheal diseases resulting from acute pathogenic infection and inflammation in the gut. This application aims at understanding post-translational modification of human NHE3. The impetus of the proposed study comes from our recent finding that NHE3s of human and primates differ from NHE3s of lower mammals, including rodents and rabbits. Nedd4-2 is an E3 ubiquitin ligase that interacts with substrate proteins via PY (PPxY) motif. We found that human NHE3 (hNHE3) interacts with Nedd4-2, which ubiquitinates hNHE3 and mediates endocytosis, and the response of hNHE3 is much greater to forskolin/PKA than rabbit NHE3. Although mice have widely been used to understand the physiological role of NHE3, mice are relatively resistant to development diarrhea. We will examine the idea that ubiquitination of hNHE3 contributes to the increased severity of acute diarrhea in man. To begin, we will compare the regulation of hNHE3 and mouse NHE3 in vivo using transgenic animals. We will study how hNHE3 is endocytosed, recycled and degraded by Nedd4-2 mediated ubiquitination. Ubiquitination is counteracted by deubiqutination. We have identified several putative deubiquinating enzymes (DUBs) that interact with hNHE3. We plan to investigate how Nedd4-2 and DUBs dynamically regulated hNHE3. Our work is expected to reveal new mechanism for the control of intestinal brush border NHE3, and identify Nedd4-2 as a novel target for the therapeutic of diarrheal diseases caused by abnormal sodium and water balance in intestinal epithelium.