Epithelial transport in digestive diseases: mice, monolayers, and mechanisms

Abstract

The transport of electrolytes and fluid by the intestinal epithelium is critical in health to maintain appropriate levels of fluidity of the intestinal contents. The transport mechanisms that underlie this physiological process are also subject to derangement in various digestive disease states, such as diarrheal illnesses. This article summarizes the 2019 Hans Ussing Lecture of the Epithelial Transport Group of the American Physiological Society and discusses some pathways by which intestinal transport is dysregulated, particularly in the setting of infection with the diarrheal pathogen, Salmonella, and in patients treated with small-molecule inhibitors of the tyrosine kinase activity of the epidermal growth factor receptor (EGFr-TKI). The burdensome diarrhea in patients infected with Salmonella may be attributable to decreased expression of the chloride-bicarbonate exchanger downregulated in adenoma (DRA) that participates in electroneutral NaCl absorption. This outcome is possibly secondary to increased epithelial proliferation and/or decreased epithelial differentiation that occurs following infection. Conversely, the diarrheal side effects of cancer treatment with EGFr-TKI may be related to the known ability of EGFr-associated signaling to reduce calcium-dependent chloride secretion. Overall, the findings described may suggest targets for therapeutic intervention in a variety of diarrheal disease states.

Keywords: Ussing chamber; chloride secretion; epidermal growth factor receptor; infectious diarrhea; sodium absorption.

Fig. 1.

Major pathways for electrolyte absorption and secretion across epithelial cells in the mammalian intestine. The left-hand side of the figure shows absorptive pathways, with the electroneutral NaCl absorptive mechanism depicted in the left-most cell present throughout the small intestine and colon, whereas the electrogenic sodium absorptive mechanism in the adjacent cell is present only in the distal colon. The right-hand side of the figure shows the electrogenic chloride secretory mechanism that is also present throughout the small intestine and colon. The central boxes depict intracellular signals that either inhibit (red) or activate specific membrane transport proteins, with their targets depicted by arrows. Note that many of the factors that inhibit absorption trigger secretion and vice versa. For both absorptive and secretory mechanisms, active electrolyte transport results in accompanying paracellular transport of water as well as (in the case of electrogenic transport mechanisms) an appropriate counterion. CaCC, calcium-activated chloride channel; CFTR, cystic fibrosis transmembrane conductance regulator; DRA, downregulated in adenoma; ENaC, epithelial sodium channel; KCC1, potassium-2 chloride co-transporter-1; NHE3, sodium/hydrogen exchanger-3; NKCC1, sodium-potassium-2 chloride cotransporter-1; TK, tyrosine kinase (e.g., epidermal growth factor receptor).

Fig. 2.

Differentiation of intestinal epithelial cells to absorptive and secretory lineages and effects of Notch and Wnt. Lgr5-positive pluripotent stem cells cycle in the crypt base under the influence of both Notch and Wnt. If levels of Notch signaling are high, committed progeny are directed towards differentiation along the absorptive pathway, ultimately resulting in mature enterocytes that express (among other markers) the chloride-bicarbonate exchanger downregulated in adenoma (DRA). Notch also suppresses the effect of Wnt that otherwise downregulates absorptive differentiation. On the other hand, when Notch signaling is relatively less active, absorptive differentiation is suppressed by unopposed Wnt activity, and differentiation to the various secretory lineages directed by Atoh1 expression, goblet cells expressing mucin 2 (Muc2), Paneth cells expressing defensing 5 (Def5), enteroendocrine cells expressing chromogranin A (CgA) and tuft cells, is promoted (44).

Fig. 3.

Working hypothesis for the promotion of diarrheal symptoms following infection of intestinal epithelial cells with nontyphoidal Salmonella spp. Bacterial invasion results in the release of soluble mediators from host epithelial cells that act in a paracrine fashion to stimulate epithelial proliferation (with accompanying immaturity of absorptive enterocytes) and/or reduced Notch signaling, promoting an overabundance of secretory epithelial lineages. The net effect is reduced expression of absorptive transporters such as the chloride-bicarbonate exchanger, downregulated in adenoma (DRA). In turn, there is a reduction of electroneutral NaCl absorption, and fluid accumulates in the intestinal lumen beyond the reserve capacity of the colon to reabsorb it (particularly when expression of the epithelial sodium channel, ENaC, in the distal colon is also suppressed), resulting in diarrhea.

Fig. 4.

Regulation of epithelial chloride secretion by the epidermal growth factor receptor (EGFr). A: the direct effect of EGF on chloride secretion involves phosphorylation and activation of EGFr-ErbB2 homodimers, binding of the p85 regulatory and p110 catalytic subunits of phosphatidylinositol (PI) 3-kinase, generation of PI(3, 4)P₂ from membrane PI(4)P, and recruitment of the ε-isoform of protein kinase C (PKC) that in turn exerts a negative effect (dashed arrow) on basolateral potassium channels. B: the muscarinic agonist carbachol (CCh) can also reduce chloride secretion by transactivating EGFr downstream of the soluble tyrosine kinases Src and Pyk-2, release of membrane-bound transforming growth factor-α by a matrix metalloproteinase (MMP), and activation of the extracellular-regulated kinase (ERK) 1 and 2 isoforms of mitogen-activated protein kinases. However, in this case, ErbB2 is not involved, and the eventual target of the inhibitory mechanism is an apical calcium-activated chloride channel (CaCC). For further details, see text.