In-vivo targeted tagging of RNA isolates cell specific transcriptional responses to environmental stimuli and identifies liver-to-adipose RNA transfer

Abstract

19Bio-fluids contain various circulating cell-free RNA transcripts (ccfRNAs). The composition of 20 these ccfRNAs varies between bio-fluids and constitute tantalizing biomarker candidates for 21 several pathologies. ccfRNAs have also been demonstrated as mediators of cellular 22 communication, yet little is known about their function in physiological and developmental 23 settings and most works are limited to in-vitro studies. Here, we have developed iTAG-RNA, a 24 novel method for the unbiased tagging of RNA transcripts in mice in-vivo. We used this method 25 to isolate hepatocytes and kidney proximal epithelial cells-specific transcriptional response to a 26 dietary challenge without interfering with the tissue architecture, and to identify multiple 27 hepatocyte-secreted ccfRNAs in plasma. We also identified transfer of these hepatic derived 28 ccfRNAs to adipose tissue, where they likely serve as a buffering mechanism to maintain 29 cholesterol and lipid homeostasis. Our findings directly demonstrate in-vivo transfer of RNAs 30 between tissues and highlight its implications for endocrine signaling and homeostasis.Little is known about the biological function of circulating cell-free RNAs (ccfRNA). Found to be 34 associated with exosomes, lipoproteins, ribonucleoproteins and more, these transcripts can be 35 isolated and sequenced from multiple bio-fluids such as plasma, lymph, cerebral fluids, breast milk 36 and more [1, 2]. ccfRNAs are directly implicated in the development of several pathologies 37 including cancer and obesity [3][4][5] and are intensively studied as disease biomarkers [6, 7]. Despite 38 this, the role they play in physiological and developmental settings and in mediating cell-to-cell 39 communication remains largely unknown. In-vitro, a growing number of works demonstrate the 40 relevance of RNA based cellular communication [8][9][10][11], however in-vivo evidence is still limited. 41This discrepancy is partly due to the difficulties posed to tracking ccfRNAs from transcriptional 42 source to potential sites of action in-vivo. Indeed the tools available to study ccfRNAs in 43 physiological settings are limited and very few studies attempt to tackle this problem directly. 44One work found evidence to suggest that the majority of circulating miRNAs originate in adipose 45 tissue and that some of the adipose derived miRNAs may play a role in the regulation of liver 46 Fgf21 levels [12]. However, this work focuses on miRNA and does not directly demonstrate 47 transfer of RNAs between tissues nor directly identify adipose secreted RNAs. 48Transfer of miRNAs was also demonstrated between epithelial cells of the caput epididymis to 49 maturing spermatozoa, leading to a shift in sperm RNA content during its maturation [13]. This 50 study made use of 4-thiouracil-tagging (TU-tagging) [14] combined with SLAM-Seq [15] to 51 demonstrate loading of miRNAs transcribed in caput epididymis into maturing spermatozoa. TU-52 tagging entails cell-type specific expression of uracil phosphoribosyltransferase...