Functional Characterization of Differentially Expressed miRNAs in Trematomus bernacchii Acclimated to Acute and Chronic Thermal Stress

The Antarctic notothenioids have evolved for millions of years in a cold stable environment and are found to have a narrow window for thermal tolerance. Global climate change is predicted to bring warmer ocean temperatures in polar regions thereby challenging the physiological capacity of these fish to environmental perturbations. Despite the lack of an inducible heat shock response (HSR), notothenioids have displayed remarkable physiological plasticity and an ability to at least partially compensate for the effects of thermal stress. Both physiological and transcriptomic studies have signaled these fish can mitigate the effects of acute heat stress by employing other aspects of the cellular stress response (CSR) that help confer thermotolerance as well as drive homeostatic mechanisms during long term thermal acclimations. However, the regulatory mechanisms that determine temperature-induced changes in gene expression remain largely unknown. Therefore, this study utilized next generation sequencing coupled with a bioinformatics in silico approach to explore the regulatory role of microRNAs in governing the transcriptomic level response observed in notothenioids with respect to the CSR. MicroRNAs (miRNAs) are small (~22 nucleotides) evolutionarily conserved, non-coding RNAs that predominantly downregulate gene expression in a sequence specific manner. Using RNAseq, this study characterized the global expression of 125 distinct miRNAs in Trematomus bernacchii gill tissue. Additionally, I examined the functional role of 10 differentially expressed (DE) miRNAs in T. bernacchii given an acute (7 days) and long-term thermal acclimation to +4 °C. Using various bioinformatics pipelines, this study determined the gene targets of DE miRNA that are enriched in particular biological pathways and examined the impact of these enriched miRNA pathways with respect to the CSR in T. bernacchii. The miRNA enriched pathways identified in these analyses were found to be involved in regulating diverse aspects of the CSR during acute and chronic heat stress in T. bernacchii such as inflammatory response, antioxidant activity, lipid metabolism, tissue and cytoskeletal remodeling, membrane maintenance, cell proliferation and apoptosis. While most miRNA enriched pathway effects coincided with the acute and chronic CSR observed in the transcriptome of thermally stressed T bernacchii, other miRNA pathways appear to potentially exert a contradictory effect.