Takagi Wataru, Kajimura Makiko, Tanaka Hironori, Hasegawa Kumi, Bell Justin D, Toop Tes, Donald John A, Hyodo Susumu
Laboratory of Physiology, Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba 277-8564, Japan.
J Exp Biol. 2014 Apr 15;217(Pt 8):1353-62. doi: 10.1242/jeb.094649. Epub 2013 Dec 20.
Marine cartilaginous fish retain a high concentration of urea to maintain the plasma slightly hyperosmotic to the surrounding seawater. In adult fish, urea is produced by hepatic and extrahepatic ornithine urea cycles (OUCs). However, little is known about the urea retention mechanism in developing cartilaginous fish embryos. In order to address the question as to the mechanism of urea-based osmoregulation in developing embryos, the present study examined the gene expression profiles of OUC enzymes in oviparous holocephalan elephant fish (Callorhinchus milii) embryos. We found that the yolk sac membrane (YSM) makes an important contribution to the ureosmotic strategy of the early embryonic period. The expression of OUC enzyme genes was detectable in the embryonic body from at least stage 28, and increased markedly during development to hatching, which is most probably due to growth of the liver. During the early developmental period, however, the expression of OUC enzyme genes was not prominent in the embryonic body. Meanwhile, we found that the mRNA expression of OUC enzymes was detected in the extra-embryonic YSM; the mRNA expression of cmcpsIII in the YSM was much higher than that in the embryonic body during stages 28-31. Significant levels of enzyme activity and the existence of mitochondrial-type cmgs1 transcripts in the YSM supported the mRNA findings. We also found that the cmcpsIII transcript is localized in the vascularized inner layer of the YSM. Taken together, our findings demonstrate for the first time that the YSM is involved in urea-based osmoregulation during the early to mid phase of development in oviparous cartilaginous fish.
海洋软骨鱼体内保留高浓度尿素，以使血浆渗透压略高于周围海水。成年鱼体内，尿素由肝脏和肝外鸟氨酸尿素循环（OUC）产生。然而，对于发育中的软骨鱼胚胎的尿素保留机制，人们知之甚少。为了解决发育中胚胎基于尿素的渗透压调节机制这一问题，本研究检测了卵生全头亚纲银鲛（Callorhinchus milii）胚胎中OUC酶的基因表达谱。我们发现，卵黄囊膜（YSM）对早期胚胎期的尿素渗透调节策略有重要贡献。至少在第28阶段，胚胎体内可检测到OUC酶基因的表达，且在发育至孵化过程中显著增加，这很可能是由于肝脏生长所致。然而，在早期发育阶段，胚胎体内OUC酶基因的表达并不突出。同时，我们发现胚胎外YSM中检测到了OUC酶的mRNA表达；在第28 - 31阶段，YSM中cmcpsIII的mRNA表达远高于胚胎体内。YSM中显著的酶活性水平以及线粒体型cmgs1转录本的存在支持了mRNA的研究结果。我们还发现cmcpsIII转录本定位于YSM的血管化内层。综上所述，我们的研究结果首次表明，在卵生软骨鱼发育的早期至中期，YSM参与了基于尿素的渗透压调节。
