The expression of human NHE3 in mouse intestine leads to increased diarrheal symptoms

![CDATA[Diarrheal disease is a frequent cause of emergency room visits and often results from altered ion transport across the gut epithelium by infectious agents. The Na+/H+ Exchanger 3 (NHE3) is responsible for the majority of intestinal electroneutral sodium absorption and is associated with many diarrheal diseases. While mice/rabbits have been used to investigate the mechanisms of diarrhea, they are less prone to develop diarrhea than humans. Recently, we have shown that human NHE3, but not mouse/rabbit NHE3s, interacts with the ubiquitin E3 ligase Nedd4-2. We hypothesize that this property of human NHE3 contributes to the increased severity of diarrhea. To investigate this hypothesis, we generated transgenic mice expressing human NHE3 in the intestine (hNHE3int) and Caco/2bb cells transfected with human or rabbit NHE3. The regulation of human and non-human NHE3 in response to forskolin (FSK) or cholera toxin (CTX) was investigated by measuring NHE3 activity and ubiquitination. We found that FSK significantly increased human NHE3 ubiquitination and the extent of inhibition of human NHE3 activity by FSK was greater than rabbit NHE3. Nedd4-2 knockdown blunted the inhibitory effect on human NHE3 but not rabbit. Consistently, inhibition of intestinal NHE3 by FSK was greater in hNHE3int than WT mice. In addition, treatment with CTX led to significantly higher water accumulation in the small intestine of hNHE3int compared to WT mice. These findings demonstrate that human and non-human NHE3s are differentially regulated, suggesting that the characteristics of human NHE3 regulation may contribute to increased diarrhea severity in humans.]]