Alkaline guts contribute to immunity during exposure to acidified seawater in the sea urchin larva

Evidence for HCO3- and NH3/NH4+-dependent pH regulatory mechanisms in the alkaline midgut of the sea urchin larva

Alkaline digestive systems are well described for some insect species and their larval stages. More recently, larvae of the members of ambulacraria superphylum consisting of echinoderms and hemichordates were also discovered to have highly alkaline midguts (pH 9.5-10.5) with the underlying acid-base regulatory mechanisms largely unknown. Using pharmacological inhibition of acid-base transporters in conjunction with ion-selective microelectrode measurements and pH-sensitive dyes, we investigated intracellular and extracellular pH regulatory mechanisms of midgut epithelial cells of a sea urchin (Strongylocentrotus purpuratus) larva. Our findings suggest that vacuolar-type H+-ATPase (inhibited by bafilomycin a1), carbonic anhydrase (inhibited by acetazolamide), anion-exchangers (inhibited by 4,4'-diisothiocyano-2,2'-disulfonic acid or DIDS), and soluble adenylyl cyclase (inhibited by KH7) play crucial roles in cellular acid-base regulation as well as midgut alkalization. Ammonia excretion rates were decreased in the presence of bafilomycin and colchicine, pointing toward vesicular [Formula: see text] trapping and exocytosis mechanism in eliminating nitrogenous proton equivalents from midgut cells. Finally, midgut perfusion studies revealed ouabain-sensitive luminal [Formula: see text] uptake, suggesting a role for Na+/K+-ATPase-mediated ammonia transport in midgut alkalization. This comprehensive pharmacological analysis provides a new working model relying on the CO2/[Formula: see text] and NH3/[Formula: see text] buffer systems for midgut alkalization in the sea urchin larva. These findings are discussed in the context of other alkalizing systems with strong implications for the conserved role of [Formula: see text] and NH3-driven mechanism of midgut alkalization across the animal kingdom.NEW & NOTEWORTHY Sea urchin larvae evolved highly alkaline conditions in their digestive tracts, and the underlying acid-base regulatory mechanisms are little understood. Here we present evidence that the process of luminal alkalization is cAMP-dependent. Furthermore, our data point toward the involvement of bicarbonate and ammonia in regulating midgut fluid pH. These results identified a novel mechanism for luminal alkalization in the digestive tract of a marine animal with strong implications for other alkalizing systems in animals.

Characterization of digestive proteases in the gut of a basal deuterostome

Digestive systems are complex organs that allow organisms to absorb energy from their environment to fuel vital processes such as growth, development and the maintenance of homeostasis. A comprehensive understanding of digestive physiology is therefore essential to fully understand the energetics of an organism. The digestion of proteins is of particular importance because most heterotrophic organisms are not able to synthesize all essential amino acids. While Echinoderms are basal deuterostomes that share a large genetic similarity with vertebrates, their digestion physiology remains largely unexplored. Using a genetic approach, this work demonstrated that several protease genes including an enteropeptidase, aminopeptidase, carboxypeptidase and trypsin involved in mammalian digestive networks are also found in sea urchin larvae. Through characterization including perturbation experiments with different food treatments and pharmacological inhibition of proteases using specific inhibitors, as well as transcriptomic analysis, we conclude that the trypsin-2 gene codes for a crucial enzyme for protein digestion in Strongylocentrotus purpuratus. Measurements of in vivo digestion rates in the transparent sea urchin larva were not altered by pharmacological inhibition of trypsin (using soybean trypsin inhibitor) or serine proteases (aprotinin), suggesting that proteases are not critically involved in the initial step of microalgal breakdown. This work provides new insights into the digestive physiology of a basal deuterostome and allows comparisons from the molecular to the functional level in the digestive systems of vertebrates and mammals. This knowledge will contribute to a better understanding for conserved digestive mechanisms that evolved in close interaction with their biotic and abiotic environment.

CRISPR/Cas9 mutagenesis reveals a role for ABCB1 in gut immune responses to Vibrio diazotrophicus in sea urchin larvae

The ABC transporter ABCB1 plays an important role in the disposition of xenobiotics. Embryos of most species express high levels of this transporter in early development as a protective mechanism, but its native substrates are not known. Here, we used larvae of the sea urchin Strongylocentrotus purpuratus to characterize the early life expression and role of Sp-ABCB1a, a homolog of ABCB1. The results indicate that while Sp-ABCB1a is initially expressed ubiquitously, it becomes enriched in the developing gut. Using optimized CRISPR/Cas9 gene editing methods to achieve high editing efficiency in the F0 generation, we generated ABCB1a crispant embryos with significantly reduced transporter efflux activity. When infected with the opportunistic pathogen Vibrio diazotrophicus, Sp-ABCB1a crispant larvae demonstrated significantly stronger gut inflammation, immunocyte migration and cytokine Sp-IL-17 induction, as compared with infected control larvae. The results suggest an ancestral function of ABCB1 in host-microbial interactions, with implications for the survival of invertebrate larvae in the marine microbial environment.

Na+/H+ exchangers differentially contribute to midgut fluid sodium and proton concentration in the sea urchin larva

Regulation of ionic composition and pH is a requisite of all digestive systems in the animal kingdom. Larval stages of the marine superphylum Ambulacraria, including echinoderms and hemichordates, were demonstrated to have highly alkaline conditions in their midgut with the underlying epithelial transport mechanisms being largely unknown. Using ion-selective microelectrodes, the present study demonstrated that pluteus larvae of the purple sea urchin have highly alkaline pH (pH ∼9) and low [Na+] (∼120 mmol l-1) in their midgut fluids, compared with the ionic composition of the surrounding seawater. We pharmacologically investigated the role of Na+/H+ exchangers (NHE) in intracellular pH regulation and midgut proton and sodium maintenance using the NHE inhibitor 5-(n-ethyl-n-isopropyl)amiloride (EIPA). Basolateral EIPA application decreased midgut pH while luminal application via micro-injections increased midgut [Na+], without affecting pH. Immunohistochemical analysis demonstrated a luminal localization of NHE-2 (SpSlc9a2) in the midgut epithelium. Specific knockdown of spslc9a2 using Vivo-Morpholinos led to an increase in midgut [Na+] without affecting pH. Acute acidification experiments in combination with quantitative PCR analysis and measurements of midgut pH and [Na+] identified two other NHE isoforms, Spslc9a7 and SpSlc9a8, which potentially contribute to the regulation of [Na+] and pH in midgut fluids. This work provides new insights into ion regulatory mechanisms in the midgut epithelium of sea urchin larvae. The involvement of NHEs in regulating pH and Na+ balance in midgut fluids shows conserved features of insect and vertebrate digestive systems and may contribute to the ability of sea urchin larvae to cope with changes in seawater pH.