• An Ex-vivo Model for Electrolyte Transport Along the Small Intestine

An Ex-vivo Model for Electrolyte Transport Along the Small Intestine

Undifferentiated or crypt-like, and differentiated or villus-like, human intestinal enteroids represent distinct points along the crypt–villus axis and can be used to characterize electrolyte transport processes along the small intestine, researchers report in the March issue of Gastroenterology. Studies of their duodenal enteroid model showed that electrogenic Na+/HCO3– cotransporter 1 in the intestinal mucosa might be targeted to treat secretory diarrheas.

Human enteroids grown from isolated intestinal crypts form mini-intestines. They retain segment specificity and recapitulate disorders of particular regions of the intestine. Enteroids cultured from patients retain genotypic and phenotype features, so can be used to study specific defects in intestinal function.

For example, alterations in epithelial transport, including Na+ absorption and anion or fluid secretion, contribute to acute and chronic diarrheal diseases. The Na+/H+ exchanger 3 (NHE3 or SLC9A3) or cystic fibrosis transmembrane conductance regulator (CFTR, the primary channel for epithelial Cl–/HCO3– secretion) can be disrupted or inhibited in enteroids.

Jennifer Foulke-Abel et al studied the functions of ion transporters active in intestinal Na+ absorption and fluid and anion secretion in human small intestinal enteroids.

They isolated crypts from intestinal tissues of patients, propagated the enteroids in culture, and induced them to undergo differentiation. The authors used multiphoton and time-lapse confocal microscopy to monitor intracellular pH and luminal dilatation in enteroids under basal and regulated conditions.

Transmission electron miscroscope images of the undifferentiated and differentiated enteroid brush border. Scale bar: 1 μm.

Transmission electron miscroscope images of the undifferentiated and differentiated enteroid brush border. Scale bar: 1 μm.

They found that enteroids differentiated upon withdrawal of WNT3A, reducing expression of crypt markers and increasing villus-like characteristics (see figure).

Expression of some plasma mebrane ion transporters also changed with differentiation. Levels of NHE3 protein increased whereas the solute carrier SLC12A2 (also called NKCC1) protein decreased after differentiation. There were no significant changes in CFTR.

Foulke-Abel et al showed that the enteroids recapitulate the absorptive functions of the intestinal epithelium. NHE3 activity was similar in undifferentiated and differentiated enteroids, but was affected by inhibitors, second messengers, and bacterial enterotoxins associated with traveler’s diarrhea.

The enteroids also performed the anion and fluid secretory functions of the intestinal epithelium. Forskolin, which increases levels of cAMP, induced swelling in undifferentiated and differentiated enteroids, although more rapidly in differentiated vs undifferentiated enteroids. This likely due to the weaker cell junctions in the undifferentiated enteroids, which allow fluid and anions to leak out. The authors state that that secretion from crypt-like and villus-like enteroids indicate that each can contribute to diarrhea.

Forskolin-induced swelling of the enteroids required CFTR and the solute carrier SLC12A2, as well as inhibition of NHE3.

Regulation of intracellular pH in enterocytes upon increases in levels of cAMP involved transport of HCO3 via CFTR and solute carrier family 4, Na+/HCO3 cotransporter, member 4 (SLC4A4).

These findings provide useful information about expression of electrolyte transporters in enteroids and the response of the upper small intestine to transporter regulation under physiologic and pathologic conditions.

Enteroids could be a valuable ex vivo model of the human intestinal epithelia because they have not been transformed, can be established and propagated virtually indefinitely from intestinal tissues of healthy individuals or patients with disorders, have gut segment specificity, and Na+/H+ exchange is retained after many passages.

Foulke-Abel et al conclude that differentiation in enteroids is controlled primarily by WNT3A. Enteroids can be used in functional analyses of undifferentiated crypt or transit-amplifying cells and more differentiated villus enterocytes.

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