ABSTRACT: MicroRNAs (miRNAs) are a class of small non-coding RNAs (18-25 nucleotides) that result in RNA silencing and post-transcriptional regulation of gene expression by binding to the 3'-untranslated regions of their target mRNAs. Mature miRNAs are widely reported in plants, animals and some viruses, which involved in the innate immune response, inflammatory diseases, cell signal transduction, and metabolism. Under temperature stress, miRNA expression is regulated to cope with the stress response. Up-regulation of 18 miRNAs and down-regulation of 11 miRNAs were found in the rat jejunum after heat treatment. 25 miRNA different expression under the cold and normal conditions were identified in zebrafish (Danio rerio). However, miRNAs involved in heat stress in GIFT have not been detected, and potentially important miRNA-signal pathway could have been overlooked. Study of the regulatory function of miRNA in heat-stress response is a crucial topic in the research of fish physiology. MiRNA expression profiles of tilapia in a variety of tissue have been studied recently by high-throughput next-generation sequencing technology (NGST). The expression levels of 218 miRNAs (e.g., miR-17, miR-29, miR-155 and miR-214) were significantly altered in tilapia kidney at 6-72h post- S. agalactiae infection. Additionally, 635 mature miRNAs were identified post-hatch day 5 in tilapia XX and XY gonads, in which 62 and 49 miRNAs showed different expression respectively (e.g., sex-biased miRNAs: miR-9, miR-21, miR-30a, miR-96, miR-200b, miR-212 and miR-7977). The gene expressions may contribute to immune regulation and stress adaptation in fish by heat stress. The levels of immune-related gene (e.g., immunoglobulin M, lysozyme, hepcidin and transferrin) expression were significantly up-regulated in turbot (Scophthalmus maximus) post-48h heat stress. HSPs play an important role in fish exposed to environmental stress. Some highly conserved HSPs could be synthesized in sculpins Oligocottus maculosus, turbot and GIFT in response to heat stress. Fatty acid composition and utilization in fish could be affected by heat stress. △6-desaturase-α and elongation of long chain fatty acids 5α (Elovl 5α) are involved in poly unsaturated fatty acid biosynthesis, which were obviously up-regulated in common carp (Cyprinus carpio) by heat stress, and help to increase fatty acid metabolism and maintain fluidity of cell membrane. However, in these studies the several genes were generated from known and functional protein-coding genes. The significance of mRNA transcriptome to heat-stress response in fish are far from clear. Thus, the study of miRNA and mRNA expression profiles may help us better understand the molecular mechanisms and signaling pathway of heat stress.
