Stumpp Meike, Petersen Inga, Thoben Femke, Yan Jia-Jiun, Leippe Matthias, Hu Marian Y
Zoological Institute, Christian-Albrechts University of Kiel, 24118 Kiel, Germany
Institute of Physiology, Christian-Albrechts University of Kiel, 24118 Kiel, Germany.
J Exp Biol. 2020 May 13;223(Pt 9):jeb222844. doi: 10.1242/jeb.222844.
Larval stages of members of the Abulacraria superphylum including echinoderms and hemichordates have highly alkaline midguts. To date, the reason for the evolution of such extreme pH conditions in the gut of these organisms remains unknown. Here, we test the hypothesis that, analogous to the acidic stomachs of vertebrates, these alkaline conditions may represent a first defensive barrier to protect from environmental pathogens. pH-optimum curves for five different species of marine bacteria demonstrated a rapid decrease in proliferation rates by 50-60% between pH 8.5 and 9.5. Using the marine bacterium , which elicits a coordinated immune response in the larvae of the sea urchin , we studied the physiological responses of the midgut pH regulatory machinery to this pathogen. Gastroscopic microelectrode measurements demonstrate a stimulation of midgut alkalization upon infection with accompanied by an upregulation of acid-base transporter transcripts of the midgut. Pharmacological inhibition of midgut alkalization resulted in an increased mortality rate of larvae during infection. Reductions in seawater pH resembling ocean acidification conditions lead to moderate reductions in midgut alkalization. However, these reductions in midgut pH do not affect the immune response or resilience of sea urchin larvae to a infection under ocean acidification conditions. Our study addressed the evolutionary benefits of the alkaline midgut of Ambulacraria larval stages. The data indicate that alkaline conditions in the gut may serve as a first defensive barrier against environmental pathogens and that this mechanism can compensate for changes in seawater pH.
包括棘皮动物和半索动物在内的原口动物超门成员的幼虫阶段具有高度碱性的中肠。迄今为止，这些生物肠道中如此极端的pH条件进化的原因仍然未知。在这里，我们检验了这样一个假设，即类似于脊椎动物的酸性胃，这些碱性条件可能代表了一种保护免受环境病原体侵害的第一道防御屏障。五种不同海洋细菌的pH最适曲线表明，在pH值8.5至9.5之间，增殖率迅速下降50-60%。使用能在海胆幼虫中引发协调免疫反应的海洋细菌，我们研究了中肠pH调节机制对这种病原体的生理反应。胃镜微电极测量表明，感染该细菌后中肠碱化受到刺激，同时中肠酸碱转运蛋白转录本上调。中肠碱化的药理学抑制导致幼虫在感染期间死亡率增加。类似于海洋酸化条件的海水pH降低导致中肠碱化适度降低。然而，在海洋酸化条件下，中肠pH的这些降低并不影响海胆幼虫对该细菌感染的免疫反应或恢复力。我们的研究探讨了原口动物幼虫阶段碱性中肠的进化益处。数据表明，肠道中的碱性条件可能作为对抗环境病原体的第一道防御屏障，并且这种机制可以补偿海水pH的变化。
