Sodium/hydrogen-exchanger-2 modulates colonocyte lineage differentiation

The sodium/hydrogen exchanger 2 (NHE2) is an intestinal acid extruder with crypt-predominant localization and unresolved physiological significance. Our aim was to decipher its role in colonic epithelial cell proliferation, differentiation and electrolyte transport.

Alterations induced by NHE2-deficiency were addressed in murine nhe2^-/- and nhe2^+/+ colonic crypts and colonoids, and NHE2-knockdown and control Caco2Bbe cells using pH-fluorometry, gene expression analysis and immunofluorescence.

pH_i -measurements along the colonic cryptal axis revealed significantly decreased intracellular pH (pH_i ) in the middle segment of nhe2^-/- compared to nhe2^+/+ crypts. Increased Nhe2 mRNA expression was detected in murine colonoids in the transiently amplifying/progenitor cell stage (TA/PE). Lack of Nhe2 altered the differentiation programme of colonic epithelial cells with reduced expression of absorptive lineage markers alkaline phosphatase (iAlp), Slc26a3 and transcription factor hairy and enhancer-of-split 1 (Hes1), but increased expression of secretory lineage markers Mucin 2, trefoil factor 3 (Tff3), enteroendocrine marker chromogranin A and murine atonal homolog 1 (Math1). Enterocyte differentiation was found to be pH_i dependent with acidic pH_i reducing, and alkaline pH_i stimulating the expression of enterocyte differentiation markers in Caco2Bbe cells. A thicker mucus layer, longer crypts and an expanded brush border membrane zone of sodium/hydrogen exchanger 3 (NHE3) abundance may explain the lack of inflammation and the normal fluid absorptive rate in nhe2^-/- colon.

The results suggest that NHE2 expression is activated when colonocytes emerge from the stem cell niche. Its activity increases progenitor cell pH_i and thereby supports absorptive enterocyte differentiation.