Fukamachi Shoji, Yada Takashi, Mitani Hiroshi
Department of Integrated Biosciences, University of Tokyo, Kashiwa-shi, Chiba 277-8562, Japan.
Genetics. 2005 Dec;171(4):1875-83. doi: 10.1534/genetics.105.048819. Epub 2005 Sep 2.
Somatolactin (SL) in fish belongs to the growth hormone/prolactin family. Its ortholog in tetrapods has not been identified and its function(s) remains largely unknown. The SL-deficient mutant of medaka (color interfere, ci) and an SL receptor (SLR) recently identified in salmon provide a fascinating field for investigating SL's function(s) in vivo. Here we isolated a medaka ortholog of the salmon SLR. The mRNA is transcribed in variable organs. Triglycerides and cholesterol contents in the ci are significantly higher than those in the wild type, providing the first evidence of SL's function in suppressing lipid accumulation to organs. Interestingly, phylogenetic comparisons between the medaka SLR and growth hormone receptor (GHR), which is also isolated in this study, in relation to GHRs of other fish, suggested that all GHRs reported from nonsalmonid species are, at least phylogenetically, SLRs. An extra intron inserted in medaka and pufferfish SLRs and flounder and sea bream GHRs also supports their orthologous relationship, but not with tetrapod GHRs. These results may indicate lineage-specific diversification of SLR and GHR functions among fish or just an inappropriate naming of these receptors. Further functional and comparative reassessments are necessary to address this question.
鱼类中的生长抑素（SL）属于生长激素/催乳素家族。其在四足动物中的直系同源物尚未确定，其功能在很大程度上仍不清楚。青鳉的SL缺陷突变体（颜色干扰，ci）以及最近在鲑鱼中鉴定出的SL受体（SLR）为研究SL在体内的功能提供了一个引人入胜的领域。在此，我们分离出了鲑鱼SLR的青鳉直系同源物。该mRNA在多种器官中转录。ci中的甘油三酯和胆固醇含量显著高于野生型，这为SL在抑制器官脂质积累方面的功能提供了首个证据。有趣的是，本研究中分离出的青鳉SLR与生长激素受体（GHR）之间的系统发育比较，以及与其他鱼类的GHRs相关的比较表明，所有非鲑科鱼类报道的GHRs，至少在系统发育上，都是SLRs。插入青鳉和河豚SLRs以及比目鱼和海鲷GHRs中的一个额外内含子也支持它们的直系同源关系，但与四足动物的GHRs无关。这些结果可能表明鱼类中SLR和GHR功能的谱系特异性多样化，或者只是这些受体的命名不当。需要进一步的功能和比较重新评估来解决这个问题。
