Evolution of extreme stomach pH in bilateria inferred from gastric alkalization mechanisms in basal deuterostomes

Abstract

The stomachs of most vertebrates operate at an acidic pH of 2 generated by the gastric H + / K + -ATPase located in parietal cells. The acidic pH in stomachs of vertebrates is believed to aid digestion and to protect against environmental pathogens. Little attention has been placed on whether acidic gastric pH regulation is a vertebrate character or a deuterostome ancestral trait. Here, we report alkaline conditions up to pH 10.5 in the larval digestive systems of ambulacraria (echinoderm + hemichordate), the closest relative of the chordate. Microelectrode measurements in combination with specific inhibitors for acid-base transporters and ion pumps demonstrated that the gastric alkalization machinery in sea urchin larvae is mainly based on direct H + secretion from the stomach lumen and involves a conserved set of ion pumps and transporters. Hemichordate larvae additionally utilized HCO 3 − transport pathways to generate even more alkaline digestive conditions. Molecular analyses in combination with acidification experiments supported these findings and identified genes coding for ion pumps energizing gastric alkalization. Given that insect larval guts were also reported to be alkaline, our discovery raises the hypothesis that the bilaterian ancestor utilized alkaline digestive system while the vertebrate lineage has evolved a strategy to strongly acidify their stomachs.The highly acidic pH of most vertebrate stomachs is believed to protect against environmental pathogens and the mechanisms of gastric acidification are well described [1][2][3] . The H + /K + -ATPase (HKA) in luminal membranes of parietal cells drives H + ions into the stomach lumen leading to an accumulation of hydrochloric acid 4 . This enables vertebrates to generate highly acidic conditions in their digestive systems. In contrast, midgut alkalization in lepidopteran and dipteran insect larvae, is energized by an V-H + -ATPase (VHA) coupled to processes that result in net export of H + from the luminal space [5][6][7] . These observations demonstrate that different gastric pH regulatory mechanisms have evolved in the animal kingdom. An interest is thus raised in regard to whether the highly acidic or alkaline digestive system is an ancestral character of the bilaterian.Although marine invertebrates were described to have slightly acidic to neutral pH in their digestive systems 8,9 virtually nothing is known regarding gastric pH regulation of their larval stages. In a previous study we demonstrated that larvae of the green sea urchin, Strongylocentrotus droebachiensis, have alkaline conditions in their digestive systems 10 . This study also demonstrated that their digestive enzymes are adapted to alkaline conditions and the digestive efficiency is reduced when gastric pH is decreased.

Transcriptomic responses to seawater acidification among sea urchin populations inhabiting a natural pH mosaic

“…These data imply strong pH regulatory capabilities for gastric epithelia surrounding the stomach, but also highlight an additional energetic sink for urchin larvae that further increases the metabolic cost of living in low pH sea water (Stumpp et al . ).…”

“…ATP demand) are caused by a shift of energy partitioning towards processes needed to maintain acid–base homeostasis (Stumpp et al . , , ; Cyronak et al . ; Pan et al .…”

“…Remarkably, ATP used for ion transport and protein synthesis, two processes central to pH compensation in larval urchins (Stumpp et al . , ), can alone consume 64% and 84% of total available ATP in prefeeding stages and unfed urchin larvae when cultured in low pH sea water, respectively (Pan et al . ).…”

“…In low pH sea water, protons diffuse from extracellular spaces into the larval stomach, and stomach epithelial cells must therefore pump protons back into the extracellular fluid to maintain alkaline stomach conditions required for optimal digestive enzyme activity (Stumpp et al . , ). Early‐life stage urchins can at least partially compensate pH within the stomach: gastric pH decreases 0.3–0.5 units during chronic exposure to seawater pH of 7.7 or 7.4, yet still remains greater than 9 under both these pH regimes (Stumpp et al .…”

“…Prevailing theory posits that increases in metabolic rate (i.e. ATP demand) are caused by a shift of energy partitioning towards processes needed to maintain acid-base homeostasis (Stumpp et al 2012a(Stumpp et al , 2013(Stumpp et al , 2015Cyronak et al 2016;Pan et al 2015). In early-life stage urchins, the space containing the extracellular matrix between the epidermis and gastric epithelium is separated from external sea water by a highly permeable layer of cells, and fine-scale measurements show pH in this region conforms to that of sea water (Stumpp et al 2012a).…”