Oscillatory Transepithelial H+ Flux Regulates a Rhythmic Behavior in C. elegans

Abstract

In C. elegans, rhythmic defecation is timed by oscillatory Ca(2+) signaling in the intestine [1-5]. Here, by using fluorescent biosensors in live, unrestrained worms, we show that intestinal pH also oscillates during defecation and that transepithelial proton movement is essential for defecation signaling. The intestinal cytoplasm is acidified by proton influx from the lumen during defecation. Acidification is predicted to trigger Na(+)/H(+) exchange activity and subsequent proton efflux. The Na(+)/H(+) exchanger NHX-7 (PBO-4) extrudes protons across the basolateral membrane and is necessary for both acute acidification of the pseudocoelom and for strong contractions of the posterior body wall muscles during defecation. This suggests that secreted protons transmit a signal between the intestine and muscle. NHX-2 is a second Na(+)/H(+) exchanger whose distribution is limited to the apical membranes facing the intestinal lumen. RNA interference of nhx-2 reduces the basal pH of the intestinal cells, reduces the rate of proton movement between the lumen and the cytoplasm during defecation, and extends the defecation period. Thus, the cell may integrate both pH and calcium signals to regulate defecation timing. Overall, these results establish the defecation cycle as a model system for studying transepithelial proton flux in tissues that maintain systemic acid-base balance.

Proton production, regulation and pathophysiological roles in the mammalian brain

“…The major role of these proton extruders is to achieve pH homeostasis which is a vital function shared by all tissues [1,22] . However, accumulating evidence shows that localized proton gradients exist within [23][24][25][26] and even between [5,6] cells. Such pH microdomains are likely to have consequences for localized receptors, channels and enzymes, which may be involved in proton signaling.…”

“…It is reported that the asymmetrical expression and activation of NHE1 in cells generates a local intracellular pH gradient and thus induces or at least supports the rearrangement of the cytoskeleton by affecting cofilin, an actin-binding protein that increases the recycling of actin monomers [1] . While a great deal of effort has been focused on the involvement of NHE1 in cell migration and polarity [33][34][35][36] , two studies have demonstrated that protons, released via C. elegans intestinal NHE PBO-4 (also known as NHX-7, the ortholog of mammalian NHE1), act on a proton-gated cation channel (comprised of PBO-5 and PBO-6 subunits) located in the adjacent muscle cells to induce contraction during the defecation cycle [5,6] . These studies provided characterized [37] .…”

“…However, the exact mechanism of endogenous activation of ASIC1a is still poorly understood. Based on the exciting discovery in C. elegans [5,6] , one possibility would be to couple ASIC1a activation with endogenous proton sources such as NHEs, V-ATPases or carbonic anhydrases (CAs) (Fig. 1).…”

“…Elimination or inhibition of pH-sensitive proteins, such as Na + /H + exchangers (NHEs) [2] , monocarboxylate transporters (MCTs) [3] , and acid-sensing ion channels (ASICs) [4] , results in seizures, ataxia, impaired acid secretion or sour-sensing in mice, suggesting that protons are involved in signal transduction or the maintenance of cellular function. In particular, recent studies in C. elegans demonstrated that protons even directly serve as transmitters during muscle contraction [5,6] .…”

Extracellular pH in Restricted Domains as a Gating Signal for Ion Channels Involved in Transepithelial Transport

“…12,14,15) Rapid and local pH o transients, and their effects on cellular processes, might be part of intra-and intercellular signaling cascades in nervous systems as well as in other tissues. [16][17][18] Understanding the physiological or pathological significance of these changes requires an improved knowledge of how the main actors of this interplay, i.e. transporters and ion channels, are regulated by pH o , and, importantly, monitoring pH o in real time with high spatial and temporal resolution.…”