The Role of Plasma Membrane Sodium/Hydrogen Exchangers in Gastrointestinal Functions: Proliferation and Differentiation, Fluid/Electrolyte Transport and Barrier Integrity

Abstract

The five plasma membrane Na⁺/H⁺ exchanger (NHE) isoforms in the gastrointestinal tract are characterized by distinct cellular localization, tissue distribution, inhibitor sensitivities, and physiological regulation. NHE1 (Slc9a1) is ubiquitously expressed along the gastrointestinal tract in the basolateral membrane of enterocytes, but so far, an exclusive role for NHE1 in enterocyte physiology has remained elusive. NHE2 (Slc9a2) and NHE8 (Slc9a8) are apically expressed isoforms with ubiquitous distribution along the colonic crypt axis. They are involved in pH_i regulation of intestinal epithelial cells. Combined use of a knockout mouse model, intestinal organoid technology, and specific inhibitors revealed previously unrecognized actions of NHE2 and NHE8 in enterocyte proliferation and differentiation. NHE3 (Slc9a3), expressed in the apical membrane of differentiated intestinal epithelial cells, functions as the predominant nutrient-independent Na⁺ absorptive mechanism in the gut. The new selective NHE3 inhibitor (Tenapanor) allowed discovery of novel pathophysiological and drug-targetable NHE3 functions in cystic-fibrosis associated intestinal obstructions. NHE4, expressed in the basolateral membrane of parietal cells, is essential for parietal cell integrity and acid secretory function, through its role in cell volume regulation. This review focuses on the expression, regulation and activity of the five plasma membrane Na⁺/H⁺ exchangers in the gastrointestinal tract, emphasizing their role in maintaining intestinal homeostasis, or their impact on disease pathogenesis. We point to major open questions in identifying NHE interacting partners in central cellular pathways and processes and the necessity of determining their physiological role in a system where their endogenous expression/activity is maintained, such as organoids derived from different parts of the gastrointestinal tract.

Keywords: NHE; NHE inhibitors; constipation; cystic fibrosis; diarrhea; differentiation; gastrointestinal tract; sodium/hydrogen exchange.

FIGURE 1

Localization of the different plasma membrane NHE isoforms in the gastric epithelial cells. A corpus gastric gland is schematically represented with focus on a parietal cell (bottom right) and a mucous surface cell (top right), and the NHEs and other ion transporters involved in acid/base homeostasis. NHE1 is found in the basolateral membrane of the surface and neck mucous cells, chief cells, and parietal cells (Stuart-Tilley et al., 1994; Rossmann et al., 2001). NHE2 expression was demonstrated in rabbit gastric mucous, chief and parietal cells (Seidler et al., 1997; Rossmann et al., 2001) and later confirmed by immunohistochemical staining in the apical membranes of gastric surface epithelium (Xue et al., 2011). NHE3 expression has been shown in the rat (Orlowski et al., 1992), human, and guinea pig (Kulaksiz et al., 2001), but not in rabbit gastric mucosa (Rossmann et al., 2001), and functionally identified in the parietal cell apical membrane (Kirchhoff et al., 2003). NHE4 is expressed in the basolateral membrane of parietal and chief cells and to a lesser extent in mucous cells (Pizzonia et al., 1998; Rossmann et al., 2001). NHE8 was described to be apically located in the mouse fundic and pyloric glands (Xu et al., 2013).

FIGURE 2

Distribution of the different plasma membrane NHE isoforms along the crypt villus axis of the small intestine. A crypt villus axis of the small intestine with a focus on the crypt (bottom right) and villus cell (top right) is schematically represented. NHE1 is the house keeping NHE isoform located in the basolateral membrane of both cryptal and villus cells of the small intestine (Bookstein et al., 1994a; Dudeja et al., 1996). NHE2 is found in the apical membrane of the intestinal epithelial cells, presumably more in the cryptal compared to the villus region (Hoogerwerf et al., 1996; Bookstein et al., 1997). NHE3 is the major brush border NHE isoform responsible for Na⁺ absorption and located in the apical membrane of the intestinal surface cells (Bookstein et al., 1994a; Grant et al., 2015; Foulke-Abel et al., 2016). NHE8 is also expressed in the apical membrane of the epithelial cells in the small intestine, proposedly along the crypt villus axis (Xu et al., 2005).

FIGURE 3

Distribution of the different plasma membrane NHE isoforms along the colonic cryptal axis in the mouse colon. Crypt of the small intestine with a focus on the basal (bottom right) and surface cell (top right) is schematically represented. NHE1 is expressed along the cryptal axis in the basolateral membrane of the epithelial cells. NHE2 is expressed in the apical membrane predominantly in the cryptal region (Chu et al., 2002; Bachmann et al., 2004; Guan et al., 2006). In contrast, NHE3 is found in the apical membrane of the surface colonic epithelial cells (Bachmann et al., 2004; Guan et al., 2006; Talbot and Lytle, 2010; Nikolovska et al., 2022). NHE8 is expressed ubiquitously along the cryptal axis in the apical membrane of colonocytes (Xu et al., 2019).

FIGURE 4

Transcriptional regulation of NHE3 expression. Numerous transcription factors including Sp1, Sp3, EGR-1 and HNF4a, as well as the stimulated glucocorticoid and mineralocorticoid receptors stimulate the expression of NHE3 by binding to promoter motifs. Phosphorylation of Sp1 and Sp3 inhibits their interaction with the promoter. For more detailed information, please see main text.

FIGURE 5

NHE3 regulation at (post)translational level and by trafficking. Dopamine inhibits the translation process and accelerates proteasomal NHE3 degradation by ubiquitination or phosphorylation. The ubiquitin-specific proteases USP7 and USP10 additively prevent protea-/endosomal degradation. Cholera toxin leads to phosphorylation of sorting nexin 27 and NHE3. It thus inhibits trafficking to the membrane and accelerates degradation, respectively. E3 ubiquitin ligase Nedd4-2 mediates removal of NHE3 from the plasma membrane by endocytic internalization without NHE3 being degraded. Please see main text for more detailed information.