A Reduction in Intestinal Cell pH i Due to Loss of the Caenorhabditis elegans Na+/H+ Exchanger NHX-2 Increases Life Span

Keith Nehrke

doi:10.1074/jbc.m307351200

Abstract

Na+/H+ exchangers are involved in cell volume regulation, fluid secretion and absorption, and pH homeostasis. NHX-2 is a Caenorhabditis elegans Na+/H+ exchanger expressed exclusively at the apical membrane of intestinal epithelial cells. The inactivation of various intestinal nutrient transport proteins has been shown previously to influence aging via metabolic potential and a mechanism resembling caloric restriction. We report here a functional coupling of NHX-2 activity with nutrient uptake that results in long lived worms. Gene inactivation of nhx-2 by RNAi led to a loss of fat stores in the intestine and a 40% increase in longevity. The NHX-2 protein was coincidentally expressed with OPT-2, an oligopeptide transporter that is driven by a transmembrane proton gradient and that is also known to be involved in fat accumulation. Gene inactivation of opt-2 led to a phenotype resembling that of nhx-2, although not as severe. In order to explore this potential functional interaction, we combined RNA interference with a genetically encoded, fluorescence-based reagent to measure intestinal intracellular pH (pHi) in live worms under physiological conditions. Our results suggest first that OPT-2 is the main dipeptide uptake pathway in the nematode intestine, and second that dipeptide uptake results in intestinal cell acidification, and finally that recovery following dipeptide-induced acidification is normally a function of NHX-2. The loss of NHX-2 protein results in decreased steady-state intestinal cell pHi, and we hypothesize that this change perturbs proton-coupled nutrient uptake processes such as performed by OPT-2. Our data demonstrate a functional role for a Na+/H+ exchanger in nutrient absorption in vivo and lays the groundwork for examining integrated acid-base physiology in a non-mammalian model organism.

Highlights

Naϩ/Hϩ exchangers mediate the electroneutral transfer of extracellular Naϩ for an intracellular Hϩ
The NHX-2 protein was coincidentally expressed with OPT-2, an oligopeptide transporter that is driven by a transmembrane proton gradient and that is known to be involved in fat accumulation
As the entire repertoire of nine potential Naϩ/Hϩ exchanger homologs has recently been cloned from C. elegans [11], we have utilized a combination of reverse genetics and a novel technique for in vivo pH measurements to assess the role of one of these isoforms in normal gut function

Summary

Introduction

Naϩ/Hϩ exchangers mediate the electroneutral transfer of extracellular Naϩ for an intracellular Hϩ (for review see Refs. 1–3). While this study was in progress, a report was published suggesting that the inhibition of dipeptide uptake caused by treatment with vasoactive intestinal peptide was indirect and manifest primarily through a modification of NHE3 activity [15]. These results suggest that a functional coupling occurs between oligopeptide uptake and Naϩ/Hϩ exchange at the apical cell membrane. It is not known whether these observations hold in vivo nor whether Naϩ/Hϩ activity is required for oligopeptide uptake in a live organism. Intestinal Naϩ/Hϩ Exchange Regulates Life Span restriction, perhaps through a reduced nutrient uptake capacity, and a concomitant increase in life span

Methods

Results

Conclusion