UC San Diego

Intestinal fluid transport is critical for mediating proper digestion, absorption of nutrients, and clearance of waste, toxins, and pathogens, all without excessive dehydration. Movement of water is largely passive and driven by osmotic gradients established by active transport of ions. Apposite fluidity of intestinal contents is achieved through tightly regulated coordination of a complex machinery of ion transporters involved in both absorptive and secretory processes. The intestinal epithelium must respond to an ever-changing luminal environment and fluid transport is regulated in response to a variety of endogenous stimuli including neuronal, hormonal, and immune mediators, as well as exogenous environmental factors such as bacterial agents, nutrients, and toxins. The epidermal growth factor receptor (EGFr) is an important regulator of intestinal chloride secretion and can be differentially activated in response to a variety of stimuli to elicit divergent effects. Furthermore, signaling through EGFr is modified in the context of the overall physiological setting, for instance in the presence of inflammation or infection. Different components of the secretory machinery are regulated in response to differential activation of EGFr. A number of diseases and conditions exhibit dysregulation of intestinal fluid transport characterized by disturbances in absorption, secretion, or both. To fully address many of these conditions, it is important to understand how EGFr activation regulates intestinal fluid transport under various physiological conditions. To this end, we set out to identify transporter targets subject to regulation by EGFr under conditions that differentially activate the receptor. We have identified Na⁺, K⁺, 2 Cl⁻ Cotransporter 1 (NKCC1), a widely expressed transporter important to chloride secretion, as being differentially regulated through EGFr activation. When EGFr is transactivated in response to the cholinergic agonist, carbachol, NKCC1 is endocytosed from its site of activity at the basolateral plasma membrane through a mechanism that requires EGFr-dependent activation of mitogen- activated protein kinase (MAPK) pathways. However, when EGFr is activated through direct exposure to the bona fide ligand, epidermal growth factor (EGF), phosphatidylinositide-3 kinase (PI3K) is recruited rather than MAPK and NKCC1 is not endocytosed. This divergent regulation of NKCC1 through differential activation of EGFr has implications not only for the regulation of intestinal ion and fluid transport and the treatment of their dysregulation, but also for a number of other physiological processes and pathologies involving EGFr and NKCC1