Sodium pump localization in epithelia.

In epithelial cells, the sodium pump, in coordination with several other ion transporting proteins and channels, acts to regulate directional water and ion flux across the epithelial barrier. This function is dependant on the polarized localization of the sodium pump to a single plasma membrane domain. In most epithelial cell types the sodium pump is found in an exclusively basolateral position. Despite the clear importance of maintaining a polarized distribution of the sodium pump, surprisingly little is known about the specific mechanisms responsible for the targeting and trafficking of the sodium pump to the basolateral surface. We briefly discuss our current understanding of factors which may act to regulate the cellular distribution of the sodium pump, including the potential role of the sodium pump beta-subunit. Several previous, studies have suggested that the expression of the beta2 isoform (instead of beta1) may cause the apical localization of the sodium pump. This appeared to be confirmed by Wilson et al. Am J Pathol, 156: 253-268, 2000 who found that MDCK cells stably transfected with the beta2 subunit express the sodium pump at the apical surface. However, careful examination by Laughery et al.,Am J Physiol, 292: F1718-F1725, 2007, showed that the apical targeting of the pump was caused by the presence of butyrate in the cell growth media and was not due to the presence of the beta2 isoform. These findings are discussed below, along with potential explanations as to how butyrate may influence the polarity of the sodium pump in epithelial cells.