Abstract
Na-K-ATPase on the basolateral membrane provides the favorable transcellular Na gradient for the proper functioning of Na-dependent nutrient co-transporters on the brush border membrane (BBM) of enterocytes. As cells mature from crypts to villus, Na-K-ATPase activity doubles, to accommodate for the increased BBM Na-dependent nutrient absorption. However, the mechanism of increased Na-K-ATPase activity during the maturation of enterocytes is not known. Therefore, this study aimed to determine the mechanisms involved in the functional transition of Na-K-ATPase during the maturation of crypts to villus cells. Na-K-ATPase activity gradually increased as IEC-18 cells matured in vitro from day 0 (crypts) through day 4 (villus) of post-confluence. mRNA abundance and Western blot studies showed no change in the levels of Na-K-ATPase subunits α1 and β1 from 0 to 4 days post-confluent cells. However, Na-K-ATPase α1 phosphorylation levels on serine and tyrosine, but not threonine, residues gradually increased. These data indicate that as enterocytes mature from crypt-like to villus-like in culture, the functional activity of Na-K-ATPase increases secondary to altered affinity of the α1 subunit to extracellular K+, in order to accommodate the functional preference of the intestinal cell type. This altered affinity is likely due to increased phosphorylation of the α1 subunit, specifically at serine and tyrosine residues.
Highlights
An essential function of the mammalian small intestine is nutrient absorption [1]
While the intestinal epithelium is composed of multiple specialized cell types including goblet cells, enteroendocrine cells, Paneth cells, and enterocytes [2], only enterocytes are responsible for nutrient absorption from the intestinal lumen
The enterocytes comprise of undifferentiated crypt cells, which proliferate and differentiate to mature villus cells [3]
Summary
An essential function of the mammalian small intestine is nutrient absorption [1]. While the intestinal epithelium is composed of multiple specialized cell types including goblet cells, enteroendocrine cells, Paneth cells, and enterocytes [2], only enterocytes are responsible for nutrient absorption from the intestinal lumen. The enterocytes comprise of undifferentiated crypt cells, which proliferate and differentiate to mature villus cells [3]. Electrolyte and fluid absorption primarily occur through the villus cells while the crypt cells are thought to be primarily secretory. Enterocytes acquire more transport properties, and are physiologically able to absorb more nutrients compared to undifferentiated crypt cells. Na-K-ATPase, a basolateral membrane (BLM) transporter, plays a vital role in regulating ionic homeostasis, cell volume and maintaining membrane potential [4].
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