5' tRNA halves are highly expressed in the primate hippocampus and might sequence-specifically regulate gene expression.

Fragments of mature tRNAs have long been considered as mere degradation products without physiological function. However, recent reports show that tRNA-derived small RNAs (tsRNAs) play prominent roles in diverse cellular processes across a wide spectrum of species. Contrasting the situation in other small RNA pathways the mechanisms behind these effects appear more diverse, more complex, and are generally less well understood. In addition, surprisingly little is known about the expression profiles of tsRNAs across different tissues and species. Here, we provide an initial overview of tsRNA expression in different species and tissues, revealing very high levels of 5' tRNA halves (5' tRHs) particularly in the primate hippocampus. We further modulated the regulation capacity of selected 5' tRHs in human cells by transfecting synthetic tsRNA mimics ("overexpression") or antisense-RNAs ("inhibition") and identified differentially expressed transcripts based on RNA-seq. We then used a novel k-mer mapping approach to dissect the underlying targeting rules, suggesting that 5' tRHs silence genes in a sequence-specific manner, while the most efficient target sites align to the mid-region of the 5' tRH and are located within the CDS or 3' UTR of the target. This amends previous observations that tsRNAs guide Argonaute proteins to silence their targets via a miRNA-like 5' seed match and suggests a yet unknown mechanism of regulation. Finally, our data suggest that some 5' tRHs that are also able to sequence-specifically stabilize mRNAs as up-regulated mRNAs are also significantly enriched for 5' tRH target sites.

Temperature-dependent small RNA expression in Drosophila melanogaster.

Temperature has a major impact on gene expression in ectotherms. But until recently, it was not clear in which way, if any, small non-coding RNAs such as miRNAs or piRNAs contribute to thermosensitive gene regulation. We have recently shown that temperature-responsive miRNAs in Drosophila drive adaptation to different ambient temperatures on the transcriptome level. Moreover, we demonstrated that higher temperatures lead to a more efficient piRNA-dependent transposon silencing, possibly due to heat-induced unfolding of RNA secondary structures. In this commentary, we will dwell upon particular interesting aspects connected to our findings, hoping that our point of view may encourage other scientists to address some of the questions raised here. We will particularly focus on aspects related to climate-dependent transposon propagation in evolution and putative transgenerational epigenetic effects of altered small RNA transcriptomes. We further briefly indicate how temperature-responsive miRNAs may confound the interpretation of data obtained from experiments comprising heat-shock treatment which is a widely used technique not only in Drosophila genetics.

Temperature-responsive miRNAs in Drosophila orchestrate adaptation to different ambient temperatures.

The majority of Drosophila genes are expressed in a temperature-dependent manner, but the way in which small RNAs may contribute to this effect is completely unknown as we currently lack an idea of how small RNA transcriptomes change as a function of temperature. Applying high-throughput sequencing techniques complemented by quantitative real-time PCR experiments, we demonstrate that altered ambient temperature induces drastic but reversible changes in sequence composition and total abundance of both miRNA and piRNA populations. Further, mRNA sequencing reveals that the expression of miRNAs and their predicted target transcripts correlates inversely, suggesting that temperature-responsive miRNAs drive adaptation to different ambient temperatures on the transcriptome level. Finally, we demonstrate that shifts in temperature affect both primary and secondary piRNA pools, and the observed aberrations are consistent with altered expression levels of the involved Piwi-pathway factors. We further reason that enhanced ping-pong processing at 29°C is driven by dissolved RNA secondary structures at higher temperatures, uncovering target sites that are not accessible at low temperatures. Together, our results show that small RNAs are an important part of epigenetic regulatory mechanisms that ensure homeostasis and adaptation under fluctuating environmental conditions.