Cold-induced retrotransposition of fish LINEs

Classes of retrotransposons constitute a large portion of metazoan genome. There have been cases reported that genomic abundance of retrotransposons is correlated with the severity of low environmental temperatures. However, the molecular mechanisms underlying such correlation are unknown. We show here by cell transfection assays that retrotransposition (RTP) of a long interspersed nuclear element (LINE) from an Antarctic notothenioid fish Dissostichus mawsoni (dmL1) could be activated by low temperature exposure, causing increased dmL1 copies in the host cell genome. The cold-induced dmL1 propagation was demonstrated to be mediated by the mitogen-activated protein kinases (MAPK)/p38 signaling pathway, which is activated by accumulation of reactive oxygen species (ROS) in cold-stressed conditions. Surprisingly, dmL1 transfected cells showed an increase in the number of viable cells after prolonged cold exposures than non-transfected cells. Features of cold inducibility of dmL1 were recapitulated in LINEs of zebrafish origin both in cultured cell lines and tissues, suggesting existence of a common cold-induced LINE amplification in fishes. The findings reveal an important function of LINEs in temperature adaptation and provid insights into the MAPK/p38 stress responsive pathway that shapes LINE composition in fishes facing cold stresses.