OAT and ODC expression in salinity challenged mussels

Mytilid mussels (Mytilus spp.) are dominant members of coastal communities in North America and distributions of closely related species are often linked to differential tolerance to environmental stressors, such as salinity. As osmoconformers, mussels initiate cellular and molecular level changes during periods of salinity variation and previous studies of transcriptomic responses to hypoosmotic exposure in blue mussels have suggested differential utilization of the amino acid ornithine. Ornithine catabolism is used to generate glutamate or proline through the activity of ornithine aminotransferase (OAT), or to create putrescine and other polyamines through activity of ornithine decarboxylase (ODC). This study examined species-specific variation in the expression of genes involved in ornithine metabolic pathways for the euryhaline mussel M. trossulus compared to the congeners M. edulis and M. galloprovincialis, which are less tolerant to salinity variation. We found a consistent, small decrease in the expression of ODC during hypoosmotic exposure in all three species, but strong, species-specific increases in OAT expression. During hyperosmotic stress, the patterns of expression of these genes reversed. These patterns suggest that proline or glutamate synthesis is important during low salinity exposure, while polyamine synthesis may be more important during hyperosmotic exposure, which is an indication of differential osmotic stress. These responses were most pronounced in M. galloprovincialis, which showed nearly 18-fold increase in OAT expression during low salinity and a 2-fold increase in ODC expression during high salinity exposure.贻贝科贻贝（Mytilid mussels，Mytilus spp.）是北美沿岸群落的优势类群，近缘物种的分布通常与其对盐度等环境胁迫的差异化耐受能力紧密相关。作为渗透压顺应者（osmoconformers），贻贝在盐度波动期间会启动细胞及分子层面的适应性变化。既往针对蓝贻贝低渗暴露（hypoosmotic exposure）的转录组响应（transcriptomic responses）研究显示，其对氨基酸鸟氨酸（ornithine）的利用存在差异。鸟氨酸可通过鸟氨酸转氨酶（ornithine aminotransferase, OAT）的催化作用生成谷氨酸或脯氨酸，或通过鸟氨酸脱羧酶（ornithine decarboxylase, ODC）的活性产生腐胺及其他多胺类物质。本研究针对广盐性贻贝M. trossulus，与盐度耐受能力较弱的同属物种M. edulis和M. galloprovincialis，考察了其鸟氨酸代谢通路相关基因的物种特异性表达差异。研究发现，在低渗暴露过程中，三个物种的ODC表达量均呈现一致的小幅下降，但OAT的表达则出现显著的物种特异性上调。在高渗胁迫（hyperosmotic stress）条件下，这些基因的表达模式发生逆转。上述结果表明，脯氨酸或谷氨酸的合成在低盐暴露过程中发挥关键作用，而多胺合成可能在高渗胁迫期间更为重要，这一现象反映了物种间渗透压胁迫响应的差异化特征。这些响应在M. galloprovincialis中最为显著，该物种在低盐暴露时OAT的表达量上调近18倍，而在高盐暴露时ODC的表达量上调2倍。