Marine Biology Research Division, Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 93092-0202, USA.
Proc Biol Sci. 2013 May 1;280(1761):20130625. doi: 10.1098/rspb.2013.0625. Print 2013 Jun 22.
Osedax are gutless siboglinid worms that thrive on vertebrate bones lying on the ocean floor, mainly those of whales. The posterior body of female Osedax penetrates into the bone forming extensions known as 'roots', which host heterotrophic symbiotic bacteria in bacteriocytes beneath the epidermis. The Osedax root epithelium presumably absorbs bone collagen and/or lipids, which are metabolized by the symbiotic bacteria that in turn serve for Osedax's nutrition. Here, we show that Osedax roots express extremely high amounts of vacuolar-H(+)-ATPase (VHA), which is located in the apical membrane and in cytoplasmic vesicles of root and ovisac epithelial cells. The enzyme carbonic anhydrase (CA), which catalyses the hydration of CO2 into H(+) and HCO3(-), is also expressed in roots and throughout Osedax body. These results suggest Osedax roots have massive acid-secreting capacity via VHA, fuelled by H(+) derived from the CA-catalysed hydration of CO2 produced by aerobic metabolism. We propose the secreted acid dissolves the bone carbonate matrix to then allow the absorption of bone-derived nutrients across the skin. In an exciting example of convergent evolution, this model for acid secretion is remarkably similar to mammalian osteoclast cells. However, while osteoclasts dissolve bone for repairing and remodelling, the Osedax root epithelium secretes acid to dissolve foreign bone to access nutrients.
食骨蠕虫是无肠的钵水母纲动物，以海底的脊椎动物骨骼为食，主要是鲸鱼的骨骼。雌性食骨蠕虫的后体穿透骨骼形成被称为“根”的延伸结构，根内的表皮下的菌胞中寄生着异养共生细菌。食骨蠕虫的根上皮细胞可能会吸收骨胶原蛋白和/或脂质，这些物质被共生细菌代谢，然后为食骨蠕虫提供营养。在这里，我们表明食骨蠕虫的根中表达了极高水平的液泡型 H(+)-ATP 酶 (VHA)，该酶位于根和卵囊上皮细胞的顶膜和细胞质小泡中。同样在根和整个食骨蠕虫体内表达的还有碳酸酐酶 (CA)，它催化 CO2 水合生成 H(+) 和 HCO3(-)。这些结果表明，食骨蠕虫的根通过 VHA 具有巨大的酸分泌能力，由 CA 催化有氧代谢产生的 CO2 水合生成的 H+ 提供动力。我们提出，分泌的酸溶解了骨碳酸盐基质，然后允许皮肤吸收骨衍生的营养物质。这是一个酸分泌趋同进化的令人兴奋的例子，这种模型与哺乳动物破骨细胞非常相似。然而，虽然破骨细胞溶解骨用于修复和重塑，但食骨蠕虫的根上皮细胞分泌酸来溶解外来骨骼以获取营养。
